Originally Posted by
flyupfront
You can book a stopover in LHR/LGW if it's available. Enter your origin and destination and the online system will prompt you to select a stopover in London, if desired. Keep in mind that if onward flights aren't available to your final destination following your would-be stopover, you'll not be able to book.
This is a discussion about booking on Avios.com (Avios.com is a separate loyalty scheme from BAEC, targeting the UK only, with its own website). The Avios.com booking engine does NOT offer this feature, the one on BA.com does.
However availability can be different between the schemes, in terms of what they can see.