Hi, I also HE abroad. I've found workborks (Letts, etc - available from Amazon) really useful, as they let me know roughly what the child would be doing in the UK at specific ages/key stages. It's like an idiot's guide to the national curriculum! Which, um, I needed.
They're not expensive, and once you know what they should be studying, you can supplement with free downloaded materials, books from a library (if you have one available), etc. There are always great teaching strategies and lesson plans online.
As for returning... Would you be going back into the state school system in the UK? How old are your DC? As Saracen says, really only an issue for GCSEs. State schools will aim to put children with their age group, and give support as necessary.
I'll add that if your children are in an international school, make sure they're learning about the host country. I know that sounds stupid, but some intl schools are so focused on teaching home country geography, history, etc, that the host country and region gets left out. Which is a waste of a good opportunity.