YABU is my reply and I agree that you are garnering pretty blunt opinions because you posted in AIBU.
If we move to the lovely town that the state school is in DD would still be able to commute to her school, there is no favouritism at play here. However we would reduce our 6th form options as it would be no longer possible to commute to the superselective.
Why don't you move to where you think you would like them to go to 6th form then? Why are you thinking of chopping and changing schools so much? Clearly, you need to move to a more urban area with a wider school choice (which is acceptable to your standards) and which also provides more scope for higher household income. I think you might as well release equity from your house and move to a cheaper one and in so doing, make sure (if possible) that you move to a much more convenient area for everything.
And would you really move them after GCSE's? There won't be too many places going at a 6th form state superselective especially if you don't live anywhere near!
All I can say is that there is a huge amount of pressure for all your DC to succeed in order to gain potential entry to (a) grammar in another county and (b) decent scholarship to a selective indie miles away from where you live for which you are actually adding to your mortgage debt. Sorry, using a drawdown facility is additional borrowing - and you clearly cannot pay your current school fees for DC1 out of your income.
It doesn't add up for me - you spend £750 per term on your childrens' school commutes? And you spend how much on multiple extra curricular activities? Bearing in mind, full fees for day schools round here are approx. £15K pa which you still need to find NET.
You definitely need a reality check! I'm afraid that it isn't possible to have it all - but at least, you do have a valuable asset to sell in order to realise your aspirations for your children.
It's still a hell of a burden to pay 3 sets of school fees (and banking on scholarships and bursaries at that!) on a household income of £60K even if you do sell your house to realise equity of say £150K.