We're moving to Edinburgh next year and my two girls (1 and 3) are currently speaking more Spanish than English (we're living in Uruguay at the moment, so it's easy). I plan to try to keep it up by speaking at home and finding other mums/kids w. Spanish as mother tongue, so if you're still in Edinburgh next year I'll find you...!
We lived in London until last year and even so Sofia was almost totally Spanish speaking, as I had a Venezuelan nanny and spoke to her only in Spanish--so I know it can be done, just takes a lot of effort.