The shop in the Castle does that. When I bought mine I spent quite a while hanging from it (& the other contenders) before deciding, & it was definitely a Good Thing.

I actually wound up getting a more gear-loop-ish harness than I expected (think it is one designed for ice climbing & so forth) because it was so damn comfy. The contrast between that & the hire harness was quite impressive.

I can't use mine at the moment (too big) or for the next couple-few months (I don't roped-climb often anyway, & can't imagine I'm going to make it anywhere other than the Arch at the outside for a while after March!), so if you'd like to have a borrow & try it out you'd be welcome to.