Don't dive BSAC 88's
I can't see anywhere in the SE of the UK where this would case an issue.
1mb equals 30ft or 10m of air pressure. So if the highest point is a 1000ft/330m (which I doubt), then you are talking about a pressure difference of .033 of a bar or .3m difference in water pressure. TBH I can't see that making a difference by the time you have dekitted, loaded the stuff in the car and driven off.
Indeed, in the Lakes after a dive*, I can't see it making a major difference either because you don't go rocketing up to top of Snake pass immediately after the dive and there is a SI interval and you are driving slowly up the hill.
Just my £0.02
* by which I mean driving over Snake pass after a dive and not conducting a dive in the Lakes. TBH I wouldn't be bothered about diving in the Tarns either, just make sure you add a little to the stops and make sure you make a slow ascent!