I've found a pattern which uses Rico esign, Mia yarn. I'm finding it quite hard to get hold of (i'm in the UK) and I don't think I like the look of it anyway.
Can I use a different yarn so long as it matches the same gauge?

As long as it has the same wearability facets I think so, yes. By wearabilty I am meaning that if the item is supposed to stretch and the original yarn did that then you will need to look for that quality *as well as* gauge in an alternative.