You could open an account with IG index, and short it there, its very easy to do.
Careful though. If they hold rates against expectations, I think the pound will probably gain a cent or two. I the minimum bet with IG is £1 per point, with each cent being equivalent to 100 points. So at £1 a point, a cent change will gain or lose you around a hundred pounds.
As these things move to quickly, it would be best to use a guaranteed stop level if the bet goes the wrong way.
If theres one thing I've learnt from spread betting, it's that nothing is as predictable as you think.
Good luck.