Upvote:3
Airside Transit (i.e. changing flights without having to enter the UK) is possible at the Heathrow, Gatwick and Manchester Airports (regardless of terminal changes).
I assume you're a Turkish citizen, and they normally Need a visa even for changing flights (a so-called direct airside Transit visa, or DATV for short). However, you have a US visa which is a valid Substitute for this.
So, you don't Need to clear Immigration, and since you have a US visa, don't Need a British DATV
And by the way, even if you did Need to clear Immigration, you could do so without a visa and stay until 23:59 the next day