I've had this problem when I was in Liverpool before. There are the three music shops right next to each other, Curlys, Dawsons and Dolphin music. Curlys wouldn't let me try a bass as I wasn't going to by there and then, yet there was a 14 year old emo kid try a rather expensive bass for quite awhile, i mean wtf?. In Dolphin I managed to get the guy to let me have a go on a bass, he was reluctant at first. But at Dawsons let let you try anything, its quite impressive.
The best service I've ever had at a guitar shop was in Florida. Shop called Georges Music (its just outside Universal Studios). They let you try anything in anything, whilst my brother was trying a MIA Fender Strat through a normal Fender Amp, I went ahead and tried an Ibanez EW Acoustic Bass and a Ibanez electric guitar through the Joe Satriani sig amp, both head and cab, worth over £2000. It was awsome