Completion date after exchange

[InspiredToBoot]

We are about to exchange on a property we are buying and want to agree on a completion date but I am wondering, can the sellers decide to not meet the completion date after we have exchanged?

No, that's the point of the contract, everything is written down and agreed. If they delay, they need you to agree and you can refuse them or charge them financial penalties.

They can pull out but it costs so much it rarely (if ever) happens. For a start they lose their deposit, and interest accrues somewhere as wells.