The forum rules don't allow for people to recommend particular brands to you (they can Private Message you recommendations though)
But in general, I have found all of the two-part moulding compounds to be quite good and broadly similar. If you google for them, you can find UK supplies that sell in larger quantities that is quite economical.
Just be sure you get the putty type rather than the liquid, pourable types.
Although - the pourable ones can be fun too.
A tip I learned from my older brother, who is into model aircraft - you can build a little trough for making silicone moudls from lego bricks. Just build the little container, use some plastercine or something at the bottom to embed your object into and create a flat surface and pour.
I've not tried using the silicone caulk to make moulds, but why not give it a go? Sometimes a bit of mess can be worth it, especially if you get lots of nice moulds at the end of it!