Be creative with routings within the same 'zone' and get an 'open-jaw' ticket.
For example, it's possible to fly from London to any US city and return from any other city on one miles ticket, with the ability to use a 2-4-1. I did this last month - LHR-LAS/SFO-LHR in Club for 100k miles and a 2-4-1. The connecting flight was all of $150/pax on Virgin America. We specifically flew back from SFO as there was no return availability from LAS.
Note: You will need to use BA.com to search for availability in each direction 'manually' (use a pen and paper to note down the details of the flights where you can find seats) and then call the EC to book. You might incur an offline service fee, but it's worth it if you can assemble a trip close to what you want.