Sounds simple but not always in practice...
"They may face a price increase, or elements of the deal they originally signed up to may change," Ofcom said.
I suspect this will have zero impact on the problem with 18 month service contracts, that many providers (eg. EE) like to offer - to the exclusion of all others.
Many providers will allow you to pay for 12 months line rental up front, at a discount to the pcm rate, I would regard this as an "element of the the deal".
However, when the line rental is due for renewal, the service provider won't allow you to renew on an annual basis, as the duration is outside of the term of the 18 month service contract, and they won't allow you to renew the service contract early, so for the last 6 months you are forced to pay the pcm line rental rate...