My boys are 14 and 8, and by BF and I seriously thought out the education angle before deciding to make this move.
The kids in the UK are grouped according to age. From what I've heard, they are a few years ahead, but if your kids are intelligent honour students (which mine are, too), they shouldn't have any problem. We've scheduled our move for before the school year so they can start at the beginning. My BF's mum - and a few others - have mentioned they've never heard of a child being "held back," as they sometimes are in the States.
The 8 year old will probably be fine - the 15 year old though -- I had concerns because mine is 14 and only has two years before he no longer would have to attend school and those "level" tests (which I still don't completely understand!) would be taken. But I figure he's a smart boy and we'll know soon enough to get him help if it's really too overwhelming.
I know others will be able to give you more info - but since our kids are the same age, I thought I'd let you know that with all I've read and heard, I feel very comfortable bringing my kids over and enroling them in school in the UK.