You can use the "Find our cheapest fare" screen set to a day return.

https://www.britishairways.com/trave...r/public/en_gb

At the moment in short term the cheapest are LGW-GLA, LGW-JER and LGW-EDI, all in the £65 area with LGW-AMS at £57.96 in January, and do-able in an afternoon / evening on a back-to-back (AMS and GLA are probably the easiest of those four though none of them are difficult). In the medium term the cheapest is LUX at £52.84 return. There may be some lingering Vueling codeshare and Iberia services which are cheaper if departing from Spain. The other thing to look out for are the Weekender fares, which use the G fare bucket if you have Expertflyer (or other such tool), which covers a proper day out in places like DUB and LIS. Main thread here:

https://www.flyertalk.com/forum/brit...rip-fares.html