I really would advise against this. It puts you in a very vulnerable position.
I agree it does seem quite risky. The problem they have is every new build developer is asking for their current house to be Sold STC before they’ll let them reserve a property. They were SSTC, but it fell through so they lost the house they reserved (along with the deposit £500 or so). This could just keep happening again and again. I’ve suggested there are probably financial arrangements available to decouple the two and be able to demonstrate that they don’t need to sell their current house to buy the next.
I should add there are time constraints for the move as they are in the military and being posted elsewhere.
PXing is one, but they are getting hung up on not getting the ‘market value’ for their current house. Obviously you won’t, but you’ve got to consider the whole transaction and all other costs, potential or actual. It might be the best overall option.
Bridging loan is another but risky and expensive.
As was mentioned here and elsewhere, switching the current mortgage to buy to let might be an option too….. I’m curious about how that would work