What happens if you dont register for utilities in UK?

I can only tell you what happened when I bought a house which had been renovated by a builder who hadn't registered for utilities. When I took over the house and did register, there was a discrepancy between the meter readings given by the owner of the house pre-renovation and the ones I gave when setting up my new account. This led the utility companies asking me for information about the builder whom they pursued for payment of the services he had used.
Further, all utility companies have a whole branch devoted to fraudulent use of utilities and they are out there on a daily basis looking for people who aren't paying their dues.

If you don't register for utilities, then you would have no power, water, sewer, etc. Companies do not turn on the utilities until they know who will be paying them.

he gets found out and back billed, likely plus interest, after the landlord rats him out.

The outgoing resident will let the utility company know when they leave; so they'll know that a new person is moving in.

If they don't have a name, the bills will be sent to the homeowner / resident until they know who that is.

When you register you have to give the meter reading, when you leave you have to do the same again...so the person who left gave their last reading and was charged for what they used... if you move in and fail to register and give the meter resding or if it is a brand new property you will be sent a bill likely quarterly from the reading they last and you are responsible to pay for all utility usage and standing charge...if you fail to then register and pay they will give you notice that the utility will be disconnected and you sued for the bill, disconnection costs and reconnetion costs