Who has control of the basement , do you have to go through someone else's flat to get to it?
Do you know why the owner is being such a pain about it?
The owner of the basement flat was or should have been aware that the meters were situated in the basement when he bought the property. I suspect it was cheaper as a consequence. Since they are there then the other occupiers have what is called an easement of necessity to get to the meters whether he likes it or not.
I would check your title documents if you have them otherwise either you or a local conveyancing solicitor can get you copies from the Land registry along with a copy of the title to the basement to see what easements exist, if any, in your favour and against him. If they are actually stated there is less room to argue.
Also you can use the fact that he is aggressive and offensive as that is threatening behaviour.
Please let me know if there is anything I have not covered. I work during the day but on the system in the evenings.