Even "shall not" does not preclude them from making it longer if they wish, only words such as "must not" or "can not" indicate a compulsory requirement.
I disagree. "Shall not" = "will not".
As I understand it, if the OP wants to exit the contract right now they can, because they haven't signed it and the service hasn't started.
If they want to get Ofcom involved, then it's on the basis that "this retail ISP offered me a 30 month contract, and they're only supposed to offer me 24 months". What resolution are they looking for? That the ISP will offer them a 6+18 month contract instead? Maybe they'll get it, but is it really worth the hassle?
The service provider will say that they tied the customer to a 2 year contract, and just offers them an additional period of free service for the overlap period with their previous ISP. (Although it sounds like the wording of the contract explicitly says 30 months total contract period).
If Ofcom says they can't do that, then they could just withdraw the overlap free period, so the OP has to pay for all 24 months - even those which overlap with the existing ISP service. That doesn't sound like a good outcome to me.