I thoroughly reccomend Cass Arts for everything you will ever need.  Their Islington store is the best one but they are scattered all over London.  They sell high quality products at amazingly low prices:

This reccomendation is suitable for all ages and levels.  They even have a 'child art' section, and they supply craft materials, easels etc etc.  They also sell porfolios now if you are at that stage in your education.

http://www.cass-arts.co.uk/shops_01.htm

I give all my students advice on which equipment will particularly be better for them, but these articles are useful as a reminder.