What would you do?

[user1487797255]

i would really appreciate input to make the right decision about a difficult decision. I'm posting in secondary as we are trying to think ahead two or three years to make a longer-term plan.
We are in London. 2 yrs ago, we moved older DD (then Yr3) from state to an all-through independent school. DD2 also started in reception. Both are doing well (older one especially so as she'd been v unhappy before). We are not so sure about the secondary part as not especially academic (and DD are both) but very happy with the school up to Yr6.
Problem is, space. Also, we've been in london 20+yrs and feel ready for change. We live in a 2-bed flat and also own a small2-bed house in seaside town on south coast. We've explored schools near there and found good prep that feeds into co-ed public school with reputation for music (both DD musical). It has places for both DD and we're trying to decide: do it or not? The way we'd cover it is to rent out London flat and relocate to seaside at least short-term.
Big pros and cons each way.
Cons: if we move to seaside town, we'd still be in small 2-bed house. (Girls would like own room at some point.) Commute to school is worse (now 6 miles, would be 15+ miles tho there's a school bus) but we could prob relocate nearer school in time, poss get a bit more space. New school is mixed and DD has loved being in girls-only class. Saturday school. Fees rise a lot at 13+ (we are self-employed so no guarantees of work). Also, both DD are now settled and wondering whether to disrupt that after only 2 years since last move.
Pros for moving out: short-term stability (rent from London flat covers early years of fees), quieter less stressful life out of city, better school and good for music. We could try for a grammar at 11+ to lessen financial load.
Couple of extra points: selling city flat not easy as we rely on it for income after retirement age (we're not young!). We don't have to be in London daily for work. We'd like to move DD asap to give time to settle before (say) Yr6.
Just not sure whether to take this out-of-town option or stay put. What would you do? Would love to hear views. Thanks for reading!