Frames are different so 1 18" frame is not going to be the same as another.
If you are 5'6" then I would suggest you are looking at a small to medium frame... a 19" frame may feel comfortable but I'd have to say it's too big for you in reality.
Important thing is to get on a bike and turn the pedals.
If you look at Evans for example you can pay a £50 security deposit and take a bike for a decent spin round. Last one I bought was given to me spotlessly clean and given back rather muddy... I didn't care and neither did they.
And now you got me looking so here's my £1000 wishlist:
Trek Superfly
http://www.evanscycles.com/products/trek/superfly-6-2015-mountain-bike-ec072377Pinnacle Ramin Five (Really like this)
http://www.evanscycles.com/products/pinnacle/ramin-five-2015-mountain-bike-ec071316And you can go into any Evans Cycles and take one of these for a spin...