OO> Go to, or call, your Dentist and ask where you can purchase dental OO> plaster/casting material there in the UK. The receptionist or Steve,
Check with local ceramics hobby centers...especially wholesale suppliers if any are located around where you live. There is a water proof plaster used by ceramics folks in working with greenware that works great for our tools. It's cheap too! I've never used an epoxy coating or done anything special when making tools with this substance.
The plaster is known as "Hydrostone", industrial strength waterproof plaster and sold for $12.95 (U.S.) per 100 lb bag.
I use normal 1" to 2" square non glazed ceramic floor tiles. I either roughly break the outside tiles with an inexpensive tile tool obtainable from the same source that sells the tiles (local building supply retailer). I'm not very concerned about getting a perfect outside "line" of tiles on the tool. If a "gap" in the outside row of tiles will require more than half a tile, I snip off one to fit, otherwise I just ignore the gap.
Mike
* RM 1.3 * If it's useless, it will have to be documented.