This document would suggest losses within in the same UCITS can be offset against gains:
I've been advised something similar to what's in the Standard Life document linked here, but it was verbal advice. I don't have a link to back it up. I haven't yet followed through on the advice so I haven't bothered to get written confirmation. It was in the context of a Conexim platform account. I was told that if I move within the same family, e.g. from one Blackrock index-tracker to another Blackrock index-tracker, or from one Vanguard index-tracker to another Vanguard index-tracker, this event would not trigger Exit Tax and the investment would be deemed continuous.