If the OP can't cancel the hotel then, as someone else suggested, hiring a car to get to the hotel (but NOT into Central London!) may well be the easiest option.

Google reckons driving time is between 50 and 65 minutes depending on the route taken... though that probably doesn't take into account rush hour traffic on the M11/M25.

It's occurred to me, though, that the whole thing is really going to be quite a logistical nightmare...:

Arriving at 6 am they're not going to be at their freshest. Have they driven on the left before? How are they going to cope with rush-hour traffic, driving on the "wrong" side of the road, after a night of little or no sleep?

Let's assume they make it to Elstree by - say - 0900. What do they do then? They won't be able to check into the hotel, and it looks like a Travelodge-type place so may not even allow them to drop off their bags. This may mean leaving everything in the car...

Have they checked if the trains are running - or if it's going to be a week-end of engineering work and rail replacement services?

And what are they going to do on Sunday? If they go into London and leave the car at Elstree it'll almost be a case of catching the train into London and coming straight back again. The only other option for Sunday would be to go for a drive somewhere else - but where?