Correct me anyone if I'm wrong on this:
If it is a UK spec car (ie. officially imported by Middlehurst) then it will have a leather interior, and 3 year P&L warranty. If it is a Japanese import - privately imported or by a company other than Middlehurst (Nissan's only official UK Skyline dealership) - then it won't have leather (unless it's been fitted aftermarket).
If its a UK spec car, chances are you wont have to worry about anything since it will have already been taken care of by Middlehurst. You probably wouldn't even need to check the history (aside from obvious things like HPI etc).
If its a Japanese import, you should ensure that all the relevant import, SVA, etc documents are present as a lot of stolen GTRs can turn up in the UK. There are a number of threads on this forum to testify to this.
SE Parts do a free chassis number check that will verify part of a car's history as far as I know.
A Jap import should have an ECU that has been converted to run on UK fuel, recalibrated speedos (kmh -> mph), etc.
There is I'm sure other stuff that I've missed that R34 owners on the board can enlighten you with
