I am a final year undegrad student and I'm a bit lost on what I should do next year! I think I want to go on to study a phd, but I have always wanted to do a gap year after uni to go travelling. Does anyone think this will cause me a lot of problems with applying for or interviewing for a phd?
Has anyone else here done this?

I did exactly that! I went travelling to Australia and Asia straight after my undergraduate degree, and applied for (science) Phd's in the UK while I was travelling in Oz. As for the interviews, I did them by telephone. It was no problem, as long as you don't use a mobile. Anyway, everything went well and I got myself a fully funded PhD . I am now in my second year of the PhD and everything is going great, so no regrets what so ever!
In fact, there were many people applying for postgrad courses while travelling, although most were applying for Master degrees to do when they got back home.
Anyway, I definitley say go for the gap year, It will be great! I'd love to do one again. In fact, I'm sort of planning another one after I finish my PhD