The Republicans would win VERY few seats in the UK. Perhaps Huntingdon and Surrey Heath.
It's not to say people wouldn't like the GOP economic policies here, far from it. On fiscal matters alone the Republicans would win huge swathes of the South and Midlands. However the Republicans would fall to pieces on social issues. Evangelical Christianity and such like would lose the GOP a good 75% of Tory votes.
The Republicans would need to swing towards the McCain wing to stand any chance of winning a UK election.
Yes and no. If you assume that, as in the U.S, the two parties were all the electorate had ever known, you'd get very different results.
I wrote quite a bit on this last night but it got wiped by the server problem :-(