In reply to TobyA: MSR do make different mixes - this is a response from them:
"I spoke with the product manager for stoves and asked about the fuel
mixtures in our canisters. Cans made in Korea uses approx 80%
Iso-Butane and 20% Propane. Cans made in Italy/UK use approx 70% Butane
30% Propane with a small % in each cartridge of Iso-butane.
Nathan Hamm
Cascade Designs Inc."
To the OP:
You can get Primus Power Gas in the UK. This contains Iso-Butane, n-Butane and Propane altho I'm not sure of the exact proportions: the Primus website says 25% Iso-Butane, 25% Propane, but the UK distributer www.rosker.co.uk says only 10% Iso-Butane, 20% Propane. If the latter is correct, this would be worse than 30% Propane of Coleman canisters.