In reply to no_more_scotch_eggs:
The main difference between a membrane layer like goretex or event, and a PU coated material like AQ2 or Hydroshell is longevity. The latter are a nylon face fabric with a waterproof coating essentially painted on. As such, after a while they will start letting water in. This is obviously dependant on how much you use it and how well you look after it.
So if you're careful a cheap shell will last you a while if you keep it clean and reproof it.
The other main differences are the tailored finish and features like more waterproof zips and more substantial hoods.
Having a good big hood will obviously keep water out much better than one that barely comes over your face without a good peak.
So basically a cheap one will keep you as dry if you're more careful with it, and keep on top of water getting in the hood and zips where a more expensive jacket will allow you to be slightly lazier if that makes sense?
Just get what you can afford. But from experience I'd say a £100 jacket is twice as good as a £50, but a £200 isn't twice as good as a £100. The increase in performance different goes down in price, but the more expensive ones are better.