I finally committed and ordered some MSW79 Black Polished wheels from
Tegiwa Imports in Stoke as they were about £40 cheaper overall compared to other sites, so 4 of the 7.5jx18 came to just under £500 delivered.
I've also just ordered some Continental winter tyres from mytyres.co.uk as they seemed to be the only place to get the new 870 P in 225/40 R18. Autoexpress tested the 870 (non P) out and they seemed a great all rounder but particularly in wet handling/grip which is what I was looking for in the UK:
Autoexpress Continental 870. They came to £116 delivered, each, without fitting, and my local tyre place have quoted £10 to put the tyres on the wheels and balance.
I also bought some VDO TPMS sensors for these wheels from Autodoc:
VDO TPMS. Before i placed the order I cross-referenced the model number on the VDO website which seems to match so fingers-crossed they'll work from the box. If not, I'll get my local tyre place to have a scan.
Total cost came to £1135