Try with PEGY (http://www.pegy.org.uk/) for support for eg/pg - a lot of the resources are US-based though.

Yahoo UK group - https://groups.yahoo.com/neo/groups/gifted-home-ed-uk/info

British Council might be a place to start - http://www.educationuk.org/usa/articles/uk-qualifications-overseas/
Open & Distance Learning Quality Council - http://odlqc.org.uk/learners

BBC, Open U, Cambridge, Oxford, and many other universities/schools have open educational resources or free stuff online which may help you. I also know this site has some UK school curriculums/materials online - http://www.goodsitesforkids.org/.

My husband is a UK citizen so this is on our mind since our son is a dual US/UK citizen and we've never shut the door on possibly returning to the UK.