Well, you are an adult. As hard as it is at time to go against your parents wishes, sometimes you just have to.
I'm lucky, in that my folks have always been very supportive regardless of what us "kids" wanted to do (so long as it was legal...)
The way I see it is this. Parents (should) raise thier children with the hope that the child grow to be HAPPY, HONEST, MORAL, ETHICAL...etc. If your going to the UK for 6 months is what will make you happy, then they are selfish to stand in your way. If they raised you well, they should trust your judgement and your decisions. IF you decide to stay on "forever" at some future point, explain to your folks that it's what's best for YOU and that you will visit, they can visit...etc.
Don't skip this chance to please your folks...you very well may regret it later. And who wants to sit around ten years from now thinking..."if only I had gotten a BUNAC"...