as others have said - yes it absolutely is possible to do this, but you have to call up; I got this done very easily through the Gold line last week (LHR-AUS-DFW-LHR), but other lines you may need to wait or explain a little more simply what you are doing...
figure out the availability for the routes & dates you want using the way-one search feature online, then call up
the city pairs need to be closer to one another than the original departure point - so your proposed trip should be fine (as should most US open-jaws: not sure whether at the edges LHR-BOS-LAX-LHR would work...)