OK, sorry it took a while.
Silicone Mould Making - Casting - Prosthetics & Supplies UK have great silicones, it's what I use for all my moulds
PS 28 Silicone Base 5KG - RTV Silicone Rubber - Silicones - Products
Its a great workhorse rubber and much cheaper than Platsil Gel 10.
Mouldlife.net sell Polytek rubbers but they're on the expensive side for what they are IMO and
Mould Making and Casting Supplies from Bentley Advanced MaterialsBentley Advanced Materials | Materials For Doing More have all the Smooth On stuff which is also over priced for the Tin based rubbers but they're convenient and Benam also sell Vytaflex 40 urethane rubber which if you use Kick-It accelerator, will give you great cowls.
The thing with all this stuff is different places have different bits you need, I can order from 5 different places depending on what I need. Sometimes its worth the extra cost to order from one place but I like to shop around.
Hope that helps and feel free to ask any more questions.