Heathrow Customs and International Connection - Different Airline/Reservation

Personally, I think you can do this with no bags with a 95% success rate ... but what is your plan if you fail? Are you willing to take that risk if you might have to buy a new walk-up ticket?

KQ (Kenya Airways) arrives at Terminal 4 so you have a terminal change to Terminal 3 for your US Airways flight. KQ might through-check your luggage, but I would not rely on that.

If they do through-check (or you have no checked bags) then you can use the internal transfer bus, which will be much quicker than exiting via immigration and taking the Heathrow Express. It takes about fifteen minutes, and runs quite frequently. Once you arrive in T3, pass through security and pick up your boarding pass at the transfer desk: no customs or immigration. You are looking at less than an hour plane-to-plane.

If you have bags, and need to collect them: 0630 at Heathrow is an extremely busy time because it is just after the landing curfew has lifted (0600). Aircraft which arrive ahead of schedule are required to wait in a holding pattern until then, so you get a lot of aircraft bunching up early in the morning. And of course, this is the best time of the day to arrive in London ready for a full day of work, so there are a lot of flights getting in at this time; immigration is always busy. Baggage delivery is slow, the public transport between terminals is busy ... I would not try this with checked bags.

You suggested that you could change your flight. There are hundreds of flights between the USA and the UK every day. I don't believe they can all be full. AA should be willing to move you to another BA/AA/US flight on that day for a small change fee. If I were you, I would get an evening flight instead.