If you are tenant the position is completely different from if you are the owner of the flat. If you are a tenant leave it alone as the previous tenants might want to go back to FTTC and need the wiring and socket and the tenancy contract almost certainly will contain a clause about this kind of thing - read it. If you are a home owner do what people say above. I did a major removal job all round our large house this summer and I won't bore everyone with the details but it involved removing loads of the openreach/BT marked boxes you describe inside the house which are openreach property but I felt happy to remove as I don't think would ever go from fibre to the premises with Community Fibre (BT don't offer it to us) back to BT FTTC.
There is nothing electric in these open reach boxes in the house (I used an electrician for different works removing a piece of electricity cable of course). You will need a screw driver and you will want to make good - fill the hole, sand over, paint after you finish
