Sounds sort of like a 'minor' subject, as you say- I know that when I went on an open day at Reading, years ago, I was told I'd have to pick 3 subjects for my first year, when they are highly unlikely to be a part of the final degree title (ie. you apply for history and end up taking archaeology and history of art in your first year too). It could be a really good opportunity- you get a chance to try out another subject, maybe something you already enjoy or something new that just sounds really interesting, but that you'd have never dared to apply for for a whole degree - I certainly know that some of the most interesting areas of research are interdisciplinarary.