Honestly, it's pretty easy. It's a good opportunity to get more involved in science- if you are looking to do sciences at uni, then it would be better to be able to say that you have the individual science results. It gives you a good, if tenuous, grounding for AS-level.
Some parts are very boring, but that's mostly in C1 and P1, which you would be covering doing double, anyways.
I managed to enter the British system at the end of year 10, and I still got caught up fine, without really any extra effort. So, if you're worried about the difficulty, don't be. It's no harder than double, it's just slightly more work. I'd go for it if you are considering studying even one science at AS level!