Other than phone number portability, which has been discussed the new rules are essentially about coordinating

To move from VM to say BT just now means
- signing up to BT
- BT install the line or reactivate existing line
- Customer phones virgin to cancel the virgin service

The new system is
- Customer signs up to BT
- BT notify virgin, who notify the customer
- BT install the line or reactivate
- BT indicate to Virgin to cancel old line

In theory this is pretty simple, but the new system will need to take into account any delays in installing the new service

To point out to the OP that not all systems are compatible and the company has no responsibility to inform the customer of that fact.

One reason I will not use sky. as always a pain to get away from

In reply to a post by flippery:

One reason I will not use sky. as always a pain to get away from

It's actually really easy, I've done it more than once when hopping around for the best deal.