Do I need to clear Customs and Immigration at LHR flying Istanbul>Heathrow>Washington, D.C.>Orlando, FL on different airlines?

Here's the itinerary:

• Turkish Airlines, Istanbul --> London, Heathrow
• United Airlines, Heathrow --> Washington, D.C. (IAD)
• United Airlines, Washington, D.C. --> Orlando, FL (MCO)

And a few facts:

• I'm a U.S. citizen with a U.S. passport
• The travel is all on one ticket
• I'll have a checked bag
• The layover is quite short (70 minutes)
• Pretty sure I'll be both arriving at and departing from LHR's Terminal 2

Will I be required to go through Immigration and Customs at Heathrow?

I'm concerned because not only is it a short layover, but I'm dealing with two different airlines. That said, both TA and UA are in the Star Alliance.

Maybe the bags will just get checked all the way through to Orlando?

But I'm not sure that even matters if I still have to get through Customs.