We had a Surgical Lt. Cdr attached to us in Norway, he looked at peoples' feet every so often and sat in a BV drinking tea all day. I suspect you'll be doing a lot of that.
On a serious note though, you would simply follow the Royal Marines around wherever they go and provide medical support to them, be this on training deployments in the UK and abroad or operations in hot and sandy places.
If medicine is your passion, you're greener than green and fitter than fit, it's an ideal choice for you.