The Soundcountrol in Derby (and whatever it was called before that) allways let people try things no hassle, me and my mates used to go in every weekend and they let us try out pedals, amps, basses/guitars except anything rare. Bearing in mind this was about 5 years ago when i was 15ish and was just with mates so it was pretty obvious we wern't gonna be buying most of the gear that was tryed out.
That's all by the by anyway as it is now closed down