I use an ISO UK layout and it can be a bit tricky to get hold of those few important keys.
[CTRL]ALT often include proper UK ISO keys in their buys which is great but they don't have any group-buys running at the moment. You could try a PM to Bunnylake and see whether they have any extra Bro, Toxic, Nostalgia, Raindrop sets available that included UK ISO keys. So far, these have all been Signature Plastics keys.
You can get the Filco doubleshots from
here. These are reported to be SA profile from Signature Plastics. The details say that these only fit Filco keyboards but it's possible that they will fit anything with Costar stabs.