I'm primarily concerned with whether or not there are communications between the relevant agencies in the US and the UK
The controlling reference for this is a treaty between the US and UK which was drafted in 2013 and entered into force last year (2014) which says in part...
CONSIDERING that the effective administration and enforcement of the
immigration and nationality laws of the United States of America and
the United Kingdom are important to protect the health and safety of
their populations, to maintain the security of their societies, and to
promote international justice and security by denying access to their
territories to persons who are criminals or security risks;
ACKNOWLEDGING that identification of individuals who are inadmissible
under their respective immigration laws enhances their ability to
facilitate the travel of bona fide visitors;
Source: Agreement between the Government of the United Kingdom of Great Britain and Northern Ireland and the Government of the United States of America for
the Sharing of Visa, Immigration, and Nationality Information
So the answer is yes, the two governments share information, not only by this treaty (and similar ones), but also by terms implicit in the Special Relationship.
But just because there's an infrastructure in place for sharing information does not necessary mean that you personally have been included in the database, they might think that you are a 'small fry' and not worth worrying about. Their priorities are to inform each other about capital crimes or an act of terrorism, and you may not meet the reporting threshold. 3 1/2 years custody is not that bad as far as felons go. On the other hand, if you are in the database, you will not be allowed to board in the US, so no problem.
Also, a felony in one country does not mean that the UK will treat it the same way. The classic example is buggery, which carries the death penalty in some places but the UK has decriminalised it and now doesn't care about it.
Basically, the question is, could I sneak one by them?
Yes, it is possible. I am tracking a case now where a felon from the US entered the UK and overstayed for two years and is now due for an enforcement action. Although that person's criminal history had been spent when he entered the UK, he will be detained and removed as an illegal entrant/overstayer (unless they decide his human rights would be violated) as part of an inland sweep. As far as computers and databases at the port go, the odds are more in your favour than not.
All of this does not necessarily mean you are good to go and it would be a mistake to leave you with that impression. A mind-boggling 73% of enforcement actions are initiated by intelligence received from anonymous sources. The bulk of these are inland operations, but astonishingly includes tip offs coming in from overseas before the person even arrives.
Also, when you arrive at the airport you will join a queue waiting for your landing interview. While in the queue, you will be observed for various triggers like flushed face, fidgeting, palpitations, stressed behaviour, and other signs that you may be trying to pull a fast one. So by the time you reach the front of the queue you may already be pegged as suspicious before you even talk to the IO. And finally, if the IO's stop indicator is checked, you can expect a very brief intermezzo in merry olde England.
Having said all of that, it is a crime to enter the UK without disclosure of material facts, even if they do not ask (deception by silence is an immigration offense). And if you have an unspent criminal history, it's a further criminal act not to disclose it. They may want to get you on that before tipping their hand just for icing on the cake.
as the answer would save me certain deportation if that were the case.
No. The UK would not deport you, there have only been about three deportations all year so far. Instead, they would remove you as an illegal entrant. There are thousands of these all the time.
I saw that your question is prompted by using up your air miles. If they decide to remove you, the airline has the option of depleting your air miles to help defray the cost of your return flight (even if you have a ticket). I have seen it happen and people tend to get very indignant about it (ironically, more so than the removal). But since you won't be flying with them again anyway, it's probably irrelevant.
