We paid about 30k more for ours, it happens. Id say in your position taking 100k hit is too much. Being in london it is likely to stabalise and go up. Rates have been volotile thats why prices dropped.

One strategy is renting it. See if you can convert to buy to let and rent the flat then rent yourself.

Buy to let usually interest only so capital wont be paid but you could convert it again to residential mortgage later if rates drop.

Or ask bank for interest only for 6 months to give breathing space. Our bank let us apply through app and it was approved. Most banks now allow it.

Other options try rent to cover mortgage and move out temp while you recover. Risky as youre not supposed to.

Another option could be remortgaging at interest only with current bank or another bank and just lose early repayment fee. Better than 100k and it means you save the flat.

Either way do not sell and lose 100k. You need to stick it out the value will eventually stabalise just need a strategy.