Obviously you can't stop them leaving but....
The S21 you have issued can have no effect during the first six months of the tenancy.
The tenants are bound by their contract to pay up to the end of the tenancy.
If they wont let you in to do the inspection there is nothing you can practically do about it.
If you suspect they have left, do not, in any circumstances, just enter the house or change the locks. That could be seen as illegal eviction. Try to agree a surrender where you are handed the keys, and get it in writing that they are surrendering the tenancy.
The deposit can be used against any damage they might have caused.
The deposit has been placed in a scheme I hope.
You should tell the tenants they are still liable for the rent up to the end of their contract or until you can re-let the place, try to get them to allow you to advertise and show people around even while they are still there, it's to their benefit as well as yours.
Get as much info. as you can, so that if they do a bunk you have some chance of tracking them down. Car registration numbers, new address, if you can get it, anything you can think of.
Don't panic, be businesslike.
And after they have gone sue them for what they owe you.
I offer no guarantee that anything I say is correct. wysiwyg