My 2p is to go and have a brief test ride after the LBS ( and that includes the big High Street players) has vaguely set up a couple of different sized frames. They should be happy to do that. If you buy off the web, you can't do that, but you will probably get a cheaper bike that may or may not fit. You will not get a free two month first service thrown in, however.
Test rides and a service are worth something.