Hey, welcome Uncle_Bimmer - to be honest I think it's pretty quiet on the UK sub-forum here.
Not sure where you're about in London you're based but BW Chiptune in West London have a decent reputation.
https://www.bmwremapping.com/bmw-chip-tuning.
Don't know if it's okay to sat this here but babybmw.net is a slightly more active UK focussed forum for the E8x series.