I actually did a similar thing, although a few years ago. I went in on the 29th of June 2003 and did my six months. Then I decided I was going to go to Ireland on a similar visa to BUNAC, but they didn't recommend going over until February since it was hard to find a job in January. So I didn't want to be illegal, so I crossed over to Calais on the ferry on 28th December, bought some wine and went back to reenter in Dover on 29th December on a tourist visa. The immigration official went away with my visa for a while, but she came back eventually and stamped me. It was a pain, but I'm glad I played by the rules then so that I'm not gonna have any trouble now with my settlement visa. Good luck!