I'm still thinking about a trip next month.

Can fly from MAN-LHR for only £150 in basic economy ET (or a far cheaper RFS)
Or book for cash MAN-LHR-XXX, XXX being the following day (books into economy plus for around £8 less)

Drive down overnight and park at a tube station (4 days + the weekend so £40?) or after work and stay in a hotel (£58+ for a Premier Inn)

Park at the cheapest LHR T3 parking for £96 (7 Avios per £ through the BA e-store app or 7.5% cashback through TCB)

Avanti to London (£143-£188 in first, a little under half that in standard)

Just worry about BA cancelling evening flights from MAN. At least driving, I'm able to go at my own leisure.
I suppose though that if they did cancel the flight(s), I could still drive down in plenty of time.