I've yet to find Colby but I can buy Monterey Jack at the cheese counter in Sainsburys. Maybe give that a shot if you have one nearby?
What I would do for some tex-mex right now! I took the bus into London last week and we passed a Chipotle near Marble Arch. I almost screamed for the bus to stop! My next trip in will be planned around lunch there. Has anyone tried it? Is it as good as in the US?
ETA: Meant to quote UKMommy.
Chipotle in London is not as good. It actually almost flat out sucks. I don't know what is wrong with it exactly, but the beans taste almost sweet. The guac is not very creamy or seasoned right...I can do better on my own!
But Benito's Hat or Mission Burritos are both AMAZING.