Passata is in just about every supermarket there is. You can find it beside the tinned tomato's, in a carton similar to what UHT milk is stored in (but a wee bit smaller)
In Morrisons, it is on the shelves with the jars of pasta sauces not with the tinned tomatoes. I buy mine in a jar (sorry Lakitu - not contradicting all your answers - well I did, but didn't mean to!)
If you can't find some, get some tins of mega cheap chopped tomatoes and push them through a sieve with a wooden spoon. Discard the sees and any bits that won't go through.