Do you both teach at the same school? Would you each get 2 school places if only one of you were teaching there? Can you discuss this with them? They are saving money at the moment by having two teachers from the same family compared to having two separate families, each with two children... (though I am quite sure their budget does not assume every teacher will have two school aged children.)
Can you afford the fees at school if it's only one child.. would obviously put a significant dent in your earning (if the school fees where we are anything to go by..), how do school fees compare to the cost of childcare for the preschool years?
You are saving £2,000 per month now, while you are presumably covering the costs of one lot of preschool childcare and when that child goes to school, you will still have preschool childcare costs... maybe the impact of school fees won't be a large as you think?
Longer term, are you covering your mortgage costs in the UK, thus paying off your mortgage, so in 10 years time, you will have considerably less to pay off and you will have at least £2,000x12x? 4 or 5 so say £100,000... what would that leave your mortgage at?
Is being mortgage free when you return a reasonable goal? What would things look like if you did have to have a mortgage when you returned, presumably you were paying this before you left?
Yes, a family of 5 costs more than a family of 4, equally there are people with much higher salaries and people with much lower salaries - you have some lovely choices.. when you make yours, remind yourself why you made them, what the positives and the negatives were and focus on the positives!