I would check eBay before making a final decision: just searched for 25mm round bases and 254 results popped up (on the US site). Checked the UK eBay site and got 352 results for the same search. Most are
not slotta-bases. Some come pre-textured.
For example, one UK source sells 50 bases, 25mm or 20mm, 2mm or 3mm thick, laser-cut MDF, for £0.99, with £4.50 shipping. More than 2p apiece, but still rather inexpensive.
1p coins for IHMN - in a game where you need to get figures into every bit of cover you can, the smaller the base the better!
2p piece: 25.9 mm
1p piece: 20.3 mm
So using standard 25/20mm bases you might gain that slight edge over your pence-based opponent!

Mike O