I had to volunteer in a related area before being accepted onto my Canadian BSW, in addition to a personal statement and interview. You can work in unqualified and related posts here and that would strengthen one's application, but that requires a work permit. Maybe a volunteer visa, if not British / EEA citizen, for a related volunteering position if acceptable by an individual uni?
However, I would think getting onto a UK social work degree would be more difficult for a foreigner, given the bursary / funding system here and those requirements, that would seem more of an issue to me.
Brigette, if you have a British social work masters, I don't see it as a problem. If you are not British / EEA, maybe you could explain to others how you did it?
MLG