You need a 50 or possibly a 52, with a M designation if you want standard arm length.
I found this one:
Dunno if you can negotiate with the seller to ship to the UK. He also requires bank transfer for payment, which isn't a problem for me but likely is a no-go for you. I could give it a shot if you like.
The fake fur collar seems to be missing from most of the ones I've seen lately. This is a bit of a problem because the collars are sized to the jacket. There's always a bunch for sale, but not a great selection of sizes.
Unfortunately the majority of what I see out there are 44 through 48 or the Mil-Tec fakes. As for the latter, beware of ANYTHING that is brand new in big sizes that doesn't show an NVA or MdI stamp. The Mil-Tec fakes are absolutely impossible to discern from the real kit except for the stamp. At least they were kind enough to not fake that part of it.