Stopovers would not be possible using BA flights since, as far as I can tell, service from the major American cities BA serves goes straight to Heathrow. Perhaps a booking by phone may be allowed, but it seems unlikely since the most direct route from one of these cities is back to Heathrow.

Since you're not using a 2-4-1, change your originating city, and you might have an option. If the return trip needs to originate near Washington, DC, you may be able to force BA to use its partner, AA, by starting your flight from Norfolk, VA, about 180 miles away.

This change may offer you the possibility of routing through Boston. Although Richmond, VA is closer, I could not get the booking engine to route a flight beginning there, even requesting that partner availability be searched. From Norfolk, I could only get the online engine to route through JFK, but it might be possible to arrange this routing through a call to BA.

Of course, if those cities were simply examples, you'll need to look for other nearby cities from the originating point of the journey back to the UK, but the principle remains that a flight from a city such as Boston, Washington, DC, New York, Baltimore, Atlanta, etc. will not include a stopover in the US while flights from Hartford, Norfolk, Albany, York, PA, and Augusta, GA (cities, in order, somewhat close to the cities on the first list although I have not checked to see if all of them are cities where flights on AA originate) should.

I hope this is helpful.