IMO the pound is in for a bad 2008. It is overvalued on a PPP and historical basis, and there are serious issues with the underlying economy that are just going to get worse - housing boom turning into a bear market, interest rates getting slashed, and the likelihood of a sharp slowdown which will exposed the heavily indebted UK consumer. Furthermore, there is little problem with inflation. All this points to significant cuts in interest rates - and given that high yield was an important driver of capital flows into the UK, this is pretty bearish. Over the last decade sterling has had a lengthy bull market which is now getting very long in the tooth. A lot of the issues affecting the dollar in the last year are going to start affecting the pound in 2008.
I think it makes sense to short the pound against a basket of non-dollar currencies. Any rallies should be shorted into, or just load up on puts and take advantage of the low implied vol of forex options. This will be a long-term trade - I could see the Euro achieving parity vs the pound within say 3-4 years. Sterling is likely to underperform even the dollar - $1.80 would still be "expensive" in economic and historical terms.