Upvote:1
In terms of validity of your visa, it is valid till the midnight of the date of expiry; but I am sure you already know this.
So, speaking from practical terms - there is nothing illegal about your itinerary. However, as we all know - visas are not guarantees of entry into a country. The immigration officer has to be convinced that you are not going to violate or have not violated previously the terms of your visa.
The final decision lay in the hands of the immigration officer; despite the practical calculations.
Therefore, I would try to explain the purpose of your stay in the UK (as you normally would), and further explain that you have a return itinerary.
In some cases, the officers can grant you a special permission / stamp which allows you to enter the country on a limited basis. They might do this, just to make sure you don't end up violating your visa terms - say in case you get stuck in traffic and aren't able to clear immigration in time; but this is usually done in emergency situations.