I used to do that too which resulted to effectively having a return journey in hand all the time.Not quite British Airways and of little interest to those living in Britain, but for the expats in the EU it is possible to save up to 20% on Eurotunnel and ferries if you book as if you live in Britain and pay with a British credit card. I've been doing that for some time now, in the good ol' days of BFG and JHQ in particular the travel agents offered 10% military discount (arf arf) they buy or bought the tickets in England and passed on a whole 10% of the 20% saved to the customer. So I merely short circuit the system, using a VPN is of course useful.
There are certain trans-atlantic routes which work out cheaper to the final destination if you go a bit of a long way around. I used to book LHR to Boston and onward to Orlando, or LHR to Raliegh Durham and onward to Florida. They used to be significantly cheaper especially for a family of four travelling - we used to save around a third off per person by not flying direct LHR/LGW to Orlando (MCO).
A colleague of the wife's who is married to a member of American Airlines cabin crew put us onto using Raleigh Durham as the gateway to the US and then flying onward from there.
It is however cheaper to book car hire from the UK as you don't pay as much in the way of taxes by booking from there - I dunno how it works, but that is what I was told. Till my UK licence ran out I always used to book car hire here through a UK based company and saved significantly over booking here and showing a US licence.