i think younger kids will do fine. my children left the UK at ages 4 and 6, spent 1 year in the US school system, then moved to France for 1 1/2 yrs in that system (on same schedule as US system), and then returned to the UK at ages 6 and 8. they have thrived and excelled in all three countries. the UK system has children starting a year earlier, so they will have a bit of 'cushion' when you move back to the US where they will return to their peer group.
we have had our children in an excellent state school in London, but getting into a good state school is much more difficult these days give the economic climate. it's not as simple as chosing a home in a good catchment area as you still have to apply to schools and be accepted.
we have always supplemented the UK schooling with a fair amount of "American" information so they won't be lost when we move back to the US system. you can find excellent help in the brain quest series in the US,