QUOTE(rachelviolin @ Nov 7 2009, 07:33 PM)

www.fullermusic.co.uk
...not always the cheapest for price, but have a huge range and I have had good service from them.
I second this. They provide first class advice over the phone if you're not sure exactly what you're looking for or if you can't choose between different editions. You can order online or over the phone and most times delivery is the next day. Among the best and most personal service I have received from any shop or service provider anywhere - music or otherwise.
I'm not sure whether you can do it online but Blackwell's music department in Oxford is very good too. They have a huge stock and if they don't have what you're looking for they can almost always get it.