There are already many questions on visa requirements to enter the UK, including transit visas and visitor visa for family members (e.g. Can a Thai national married to a UK citizen enter the UK freely? and Non-EU spouse of an EU citizen - is visiting EU without needing a visa possible?).
Some of the rules seem very complicated. Where can I get authoritative information about that? Should I call the embassy like many people seem to advise?
In 2006, UKVI (previously UKBA, previously BIA, previously IND, etc) piloted a program of using a 3rd party contractor to interact with end-users who had questions. This was a favourable move for two reasons:
The programme was made global in about 2008, and now consular staff are not allowed to engage the public directly as a source of immigration advice. The two issues I mentioned above were, to a large extent, solved by this. However, it left end-users in a deplorable state.
Consulates installed a phone system that 'ring fences' the ECO and his staff from contact. It's a complex maze (some say bordering on demented) of menu options that lead the end-user around in circles. The end-user will spend a half-hour or so navigating the phone menu and finally arrive back at square 1. Because of this, end-users who are 'veterans' of the process will tell you don't even TRY to call the consulate. People who suggest you call the British consulate haven't got a clue. You will only get frustrated and waste your time.
The 3rd party contractor operates a pay-by-the-minute scheme where you ask a question and receive an answer. However, what happens behind the scenes is that the contractor will look up the question on Google and find the appropriate text on UKVI's site and read it to you. If you didn't understand, or it did not answer your question, they will read it to you again. To be fair, they have a few scripts that they can read covering some of the corner cases.
What else can you do? You can look it up on Google and see what the internet has on it. This works in a lot of cases, but you need to be real sure to check the date of the article and the publisher's credentials. Also be sure to sync up whether the answer is for visa-nationals or non-visa-nationals. Respondents tend to leave out lots of relevant information and when you read something like "We had no problems at all", there is usually something missing or it took place in a different legislative era than today's.
So where can you get authoritative advice? two places: from a member of the Law Society (i.e., UK solicitor) or an accredited practitioner (i.e., reguated adviser). In the majority of cases, a single telephone consultation can clear things up. The usual cost is between 50 and 125 pounds. Yes, it costs. But it is the only avenue to get absolute authoritative advice on an end-user's circumstances. Good advice can save you money because application fees are not refundable if you are refused. This is the answer to the OP's question: solicitor or regulated adviser.
Just like anything else, there are good ones and bad ones. It's foolish to take a recommendation from somebody you do not know who uses a forum; in many cases it's the solicitor/adviser using a sock-puppet to promote himself. So get a recommendation from somebody you know and trust. I would not give one here on SO because it may be interpreted as pimping out a buddy for a commission (lots of that goes on also).
If you don't know anybody, then you can search the Law Society's site for a solicitor or, the OISC site for an adviser. Advisers are mostly just as competent (sometimes more competent) as solicitors and significantly cheaper.
As @Gayot Fow explained, the chances of getting an answer from a British consulate are nil. You can certainly contact a solicitor or an adviser to get an authoritative answer. Yet, for most basic cases other than some very peculiar situations the information is present on the UKBA website. They recently added a very simple 3-step guide to find out if you need a visa:
https://www.gov.uk/check-uk-visa
There may be additional questions for some purposes of visit - but it's rather very straight forward.
Yet, if your case is complex, involving multiple visas or changing the type of visa, or visiting UK for multiple purposes, etc. then a solicitor or an immigration advisor is the way to go.
FWIW in my experience, disturbingly:
the only way to obtain (reasonably) certain information on these issues - and this includes both immigration-leaning visas and even (incredibly) just tourist-leaning visas - is, unfortunately, to actually hire an immigration-specialist Solicitors, likely in London.
There are any number of well-known, indeed famous, practices in this field so no need to list any here.
(Indeed, the field is so thriving there are sort of two poles .. those that lean towards human-rights-overstay issues, and those that lean towards, err, the fact that London is the world's general and main criminal-haven and tax-haven for everyone other than those who "unfortunately" hold a UK passport. (In the sense that MC is the primary tax haven for everyone other than those who "unfortunately" hold a French passport.) You would google things like "leading solicitors London immigration overstay" as the keywords to find newspaper articles etc, on the (many) specialists in all this, and find them that way.)
If you're a Russian/Chinese/Ukranian/Australian etc criminal/rich-person/wealthy-family etc, these specialist solicitors are almost unbelievably expensive. If you're an ordinary family who "wants the wife or kids to live in Birmingham," or indeed an actual human-rights case, they are merely expensive.
I guess, the overall UK situation in the world regarding visa is .. complex. First you have the whole colonial-era aspect with situations as complex as the whole HK thing; and latterly you have the whole "we're in the EU but not" situation; then as an overwhelming political undertow you have the whole "too many immigrants?!" issue and the confusing "we suck up to the US and fight in Iraq but we fear radical clerics" hot button issues. Also note that, astoundingly to many other nationalities, there are "different types" (!) of UK (err .. GB, whatever - nobody even knows what the country, or whatever it is, actually "is") passport, which to repeat is fairly amazing if you're used to undeniable-like citizenship concepts. This paragraph is merely a guess at why "immigration/visa issues are so dicey" in the UK/GB - but the fact is they are.
But yeah since (a) the infamous "outsourced! phone system" and the, almost incredible, (b) "oh embassies are not 'customer facing'" (i.e., "You can't ask the embassy WTF!!") as far as I know the only realistic thing to do is pay money to one of the immigration-leaning solicitors. It essentially puts People in the same category as corporations who traditionally have legal professionals "fix" all visa problems.
