That would be great if there was a magic mirror that lets you see what you look like a bit thinner and a bit bigger. Mind you, I need no motivation to eat so I wouldn't have a problem getting bigger.
To answer your question, I would want to look like the PM. Not only would I use this to lower the price of petrol and food, I would arrange for Northern Rock to scrub out my (the real me) mortgage and make sure the UK has one public holiday once per month.
I was going to say I would also take advantage of the opportunity to see what it's like to have a mans bits and bobs, but on second thoughts, maybe not while I'm GB! ewww.