Tricky ... but I think I'll move this thread to the installation issues forum. Welcome, anyway.
As for the Virgin Media street cabinet that turns out to be just inside your boundary, I think you could be in a little bit of trouble here. If it's been there for a long time it's possible that they have acquired a 'wayleave' - a kind of permission - for it to be there, simply because no-one has ever objected. For a wayleave to arise, it would need to be there for a certain length of time, but I don't know how long.
Even if no wayleave has arisen in this way, a previous owner of your house may have granted them a wayleave to put it there. If they did, then VM should be able to produce written proof of this.
You are probably going to have to involve a solicitor if you really want to get the thing moved. It is an expensive piece of equipment and they aren't going to shift it voluntarily.