Hello, I am a lawyer with over 15 years experience. I will try to help you with this.
Your neighbours can cut the hedge to the extent that it overhangs their gardenbut have no right to touch the parts of the hedge on yur side of the boundary.
There is no reason why you cannot erect a fence on your property. There is nor such thing as a right to light so I wouldn't worry about that at all.
You don't need to inform them unless the workers need access to their property t carry out work. Whether it is a good idea to tell them in advance is a judgement call for you.