If it's entirely on their land they can do pretty much what they like. It's convention to give your neighbour the good side, but not law.
As ethel says, check the land registry entry - I believe you can do this online for about 2 or 3 quid. A fact that many people don't realise is that generally speaking, even if the fence is yours that doesn't mean you have to maintain it (unless it's dangerous) or even provide it. You could just take it away and tough luck if your neighbours don't like it.