Overall, this is seen to be abusive because you are attempting to get more leave than was issued to you (whether you intended to or not).
The controlling technical reference for your question is found in Paragraph 30C of the Immigration Rules which says...
An Immigration Officer may cancel an entry clearance which is capable
of having effect as leave to enter if the holder arrives in the United
Kingdom before the day on which the entry clearance becomes effective
or if the holder seeks to enter the United Kingdom for a purpose other
than the purpose specified in the entry clearance.
You would be in breach on both conditions. What this means is that the Immigration Officer has the right to cancel your entry clearance and send you back to Australia to make a fresh application. They can also contact your sponsor and tell them you are attempting to abuse your YMS visa. I have seen it happen.
The IO does not have to remove you if you have a convincing excuse. Acclimating yourself to the UK would be a weak excuse (i.e., "a purpose other than the purpose specified in the entry clearance") and looking for a flat will raise alarm bells... Depending upon where you are located in the UK vis-a-vis the on-going roll out of the new 'right-to-rent' regulations may affect the legality of a tenancy agreement.
So while your strategy is not illegal, it is likely to be seen as abusive without a really good reason. And if they are not satisfied with your excuse, they can remove you. If they let you in on 10 August, you will need to leave the UK on 10 August two years hence to avoid overstaying (even if your visa says 16 August).
In commentary, you added that you hold a YMS visa and that you have (prudently) discussed the strategy with your sponsor. Your sponsor thinks a channel hopping strategy may work; I'm not necessarily in agreement and suggest they confirm it with their solicitor.
For your second question, about asking the consulate to change your start date, it depends upon the information in your certificate. They may decide you need a new certificate or may adjust your start date. It's impossible to accurately predict without seeing what your sponsor sent you.