Freeholders and electricity metres

I’m not sure if you are talking about a Freeholder or a Landlord?
You use both terms.

Do you own your flat? Or do you rent it?

I’m sorry but some of your other statements also seem confused:

… racked up a £3k-plus bill on his landlord supply account - I think he wants to block access to that account.

If it’s the landlords account then no one else could access it, so there is no need to block it.

… he once put the communal lighting on to a single flat’s account.

You can’t just put one metered supply onto a different meters account.

So the only way to do what you say with the communal lighting would be to physically wire the communal lighting into a flats metered supply. (And yes that does sometimes happen. usually that flat will get a discounted rent for that reason).

You don’t actually ask a question,  but I think that you are wondering if the Freeholder/Landlord can legally lock the meter cupboard.

The answer to that is - Yes.

If is not unusual for meter cupboards to be locked by the landlord, to prevent anyone (not just tenants) from tampering with the meters. And yes it can cause access problems.

One way around that is to get a smart meter fitted, if the energy account for your flat is in your name then a landlord can’t legally prevent you from getting a smart meter without good reason, and would have to give the meter engineer access to the cupboard to change the meter.

https://www.ovoenergy.com/help/article/installing-a-smart-meter-in-a-rented-property

Once you have a working smart meter then you don’t usually need regular access to it because it sends the readings automatically.
Depending on where your flat is in relation to the meter cupboard then you may be able to see the readings on you In Home Device (IHD).

As Nukecad has said in their reply some of the information is a bit confusing so hopefully you can clarify this. While the landlord has a right to put a lock on a cupboard outside of your flat this could cause an issue if the supply isolators are in this location. If a fire or other emergency happens its important that the supplies can be isolated. Failure to allow access for this could have consequences for the building owner with regards safety.