J T Batchelor (http://jtbatchelor.co.uk/) in Islington have a vast range of cast brass buckles, all made by a foundry in Walsall, and at a much cheaper price than Le Prevo or Abbey. If you are close to London, a visit to their shop/warehouse would be an eye-opener!
I have attached their buckle catalogue, and also their tools catalogue, and although I know that a lot of their tools are sourced from J Dixon in Walsall, I cannot say that all of their tools are British-made. Speak to Ted or Rocky, who will soon put you on the right track!
I should add that the last two figures in the buckle numbers (eg 25, 30, 32 etc) are the strap widths in millimetres.
buckles.pdf
Tools.pdf