I have no idea about the UK, but in the US if anything hangs over on your property you'd better do something about quickly otherwise you may lose the right to fix it later.
Example: If a neighbor builds a new fence and it's 6 inches over the property line on your side and you let it go, with no action or attempt to get it corrected, you have essentially abandoned your rights to that portion of your land. This doesn't mean you have to sue in court, but you have to do something. Certified letters to your neighbor, official complaints to the proper authority or if you must, sue in court.
There are some exceptions when it comes to vegetation, ( trees, shrubs or whatever ). Anytime they encroach on your property you can cut off the portion on your property with impunity... regardless of the consequences to the vegetation.
ETA: Now, if you buy a property with a structure that overhangs your property line and it's been there for years with no action from the seller of your property, you're screwed.