Cris - very much a matter for them to discuss/agree upon. Anyone can apply but the timing of an application to cancel or withdraw a restriction is down to the conveyancers involved in the sale/purchase
I guess if your conveyancer won't complete and apply then the ball is in the seller's court as to whether they apply and then complete on the sale
Practice guide 19: notices, restrictions and the protection of third-party interests in the register - GOV.UK
BM361240