There's no need to go abroad just for the sake of it. It isn't like Africa (or any other similar place) is the only place in the world which has problems, we've got problems here. You're in London, why not spend time volunteering for one of the many charities struggling to help the huge numbers of homeless people living on the city's streets? Combine that with some work as a HCA and I would've thought you'd be in a better position than those who spent their time abroad, I mean you'll have seen the problems faced in the UK, where ultimately you will be practising. And remember, the admissions staff are looking to see what you got out of your experiences, not just what you did. My work experience was particularly poor in the sense that I didn't have much hospital experience (about two and a half days), yet I did other things that focused on communication with different groups of people, something that will be seen as a valuable experience for a future doctor