Poor you, ScouseMum: I don't envy your decision, but the fact that you are on here trying to get informed and work out what is best for them is a sure sign that you are a great mum, and they can be thankful to you for that (although they probably won't be, right?!).
FWIW, I would advise you to go to Scotland, if you can get your kids into a decent secondary school there.
The GS system in Kent is fraught with issues and even people who have lived here for years find it an absolute minefield! ProfessorL is right that West Kent is much worse than East Kent, but even so it can be a bit hairy, and some of the state comps are not fantastic, should any of your kids not pass the 11+.
I am thinking that if your kids are moving to a completely new area, where they won't know anyone at first, then they might appreciate the fact of all going to the same school, and being there for each other, which might well not happen in Kent. (As you will probably have seen, some kids have to travel on the bus for miles to get to school, which can be unsettling if you already know the area, let alone if your new to a place.)
Also, I might be wrong, but I think you would get more for your money in Scotland house-wise?
I would say: buy some good raincoats and head north!