Try Dave Bates at http://www.stilesandbates.co.uk - you wont find much info about planks on their website - its mostly about woodturning supplies but they do sell machined or sawn hardwood planks and do cutting lists. They are a really friendly family firm and small orders are no problem.
a lot will depend on how you want your wood served.
as highlighted jwtimber is good geography wise
if you want choice and are doing your own prep - timberline in Tonbridge have an absolutely amazing selection: http://www.exotichardwoods.co.uk/ and are incredibly helpful (I will be converting the 6"x2"x5' piece of BE Maple I picked up yesterday into a desk top later...)
for a more machined and sized choice of oak you have Alsford timber who will have a local store to you for small amounts - http://www.alsfordtimber.com/corporate/ ... cator.html