I have a fantastic jeweller who is based out of London (so you don't pay ridiculous London prices) but he's happy to travel up to London and bring pieces with him if you have an idea of what you're looking for and your budget.
He and his partner are ex Burlington Arcade jewellers so they really know their stuff and specialise in antique pieces (if your brother is on a tight budget - you get a lot more bling for your buck if you buy antique and much more original too!).
We had lots of fun looking/trying on to narrow down choices to a few styles and then bought my engagement ring from him - 1920's/Art Deco and I absolutely love it.
Neil @ The Warminster Jeweller
www.thewarminsterjeweller.co.uk