Hello conshaldow..the £57 outbound is available for tomorrow,Tuesday, on all three BA flights, so it is possible to get the cheapest fares on this route the day before travel, though it *is* more one way.
The tube fare to central London is around £4 so that’s a fairer comparison than a cab (and often quicker), particularly for a Domestic flight when luggage is unlikely to be a problem when on public transport.
Either way, I get 2x32kgs check allowance on Dom, plus a sizeable rollaboard and laptop/personal item.
Heathrow security (at T5 South, using Fast Track) really isn’t the Hell it’s made out to be – I cannot recall queuing more than five minutes, except for one busy Bank Holiday Friday early on its opening months.
£114rtn is excellent value on a Dom service, and especially good value vs. the train if you’re not heading into the town centre.