I am glad you asked this question! I too am from the UK and have found it difficult to get hold of the stuff Owen mentioned. However - I have been told that the pure Vitamin C powder from Holland and Barratt is great.
I bought some the other day and I have been using a teaspoon of it in water every day. This equates to about 5000mg though which sounds like a heck of a lot - not sure if this is OK!