As Ethel indicates, governments and banks both pay people with degrees in economics (and often with vast experience in currency dealing) to try to predict such things. Even they don't generally do much better with their predictions than the man in the street.
For what it's worth, my analysis of the current economic situation (largely based upon listening to a lot of 'experts', from around the world, on BBC World Service) is that foreign investors can currently see little reason to invest in UK government bonds (or in UK businesses). So that means that they'll not be buying sterling currency, which will push its value down against most other currencies.
At typical tourist rates, �1 currently buys you about �1.04. My guess is that, in a year's time, �1 will only buy about �0.70
Chris