I had one of the most fun mornings of my life with Vivek at Emdico: http://www.emdico.co.uk/
This was a few years ago, so it may have changed but I called Emdico for an appointment and, since I wanted to choose the ring secretly for my wife, I took a day off work. The door was tricky to find, with just a small buzzer. When I was buzzed in I was in a small dark hallway with just a lift. I went up in the lift and had to press another buzzer to get into reception.
I was then showed into an office with a big leather armchair in front of a dark wood velvet-covered table. Vivek asked what sort of stone I wanted and I gave him a rough idea and budget.
He went into a store room and brough out a small box full of paper envelopes. He picked out about 5 envelopes and, from each, poured about a dozen diamonds onto the tabletop.
He gave me a magnifying glass and I spent about half an hour playing with all the different stones. I chose one, and he then showed me catalogues of bands and fittings and asked me to choose styles and materials.
I chose what I wanted the ring to look like (all included in the price of the stone). He said he’s get it made and I should collect it 2 weeks later.
Went back, picked it up, proposed and my now wife loves it.
Felt like James Bond for the day. Highly recommended!