Currently that will be LHR-LUX, £59 return, via T3, for some reason no-one wants to go there. There are currently 2 flights a day, so you could do it in day since it is back to back-able.

More usually it's LGW-JER. Cheapest is £68 return. There are a series of other LGW flights which are within a few pounds of this too, BOD, AMS, even places like Porto in the £70 range. Given it is out of season, an evening in OPO would be both interesting and cost-effective, though OPO is also back-to-backable, as is AMS and indeed BOD at a pinch.

BA.com has a page which assists you to consider your options, namely Find Our Cheapest Fares, off the Flights and Holidays tab.