Here's a list of local and internet suppliers that I've either used my self or have been recomended by students:
Art Shops: These two shop's will give a discount to students.
The Strand Gallery, Southchurch Road, Southend
Rayleigh Art shop. Berry's arcade, Rayleigh.
The first three I use on a fairly regular basis and are all good. If you spend over a certain amount, usually between £40 - £50 you get free postage. Artesaver is the retail site for Seawhite art materials.