Based on experience using this product in the US, I would also recommend Smooth Cast 300 for resin. Smooth On Oomoo 30 is a good RTV silicon rubber molding compound too.
Both appear to be available in the UK from a couple sources:
Benam.co.uk
Amazon.co.uk
I recommend the trial size as a good deal to start with. It is relatively inexpensive and will still provide enough resin to cast a decent number of items. My recollection is that I cast about five 5" trees and 80 bases ranging in size from 30-50mm from the contents of the trial size Smooth Cast 300.
While I am sure there are better products, Smooth Cast 300 was easy to use and reliable. I mixed by eye and the only problems I had were the result of failing to mix the resin well enough before pouring into the rubber mold.
Good luck!
Edited to answer question asked and incorporate other responses.