The Prescription Act 1832 provides that a right to light to a window in a building can be acquired by actual enjoyment of the right for a period of at least 20 years without interruption.
It is possible that if the prescriptive light reaching the owner's windows is interrupted or reduced by the growth of neighbours trees he may be entitled to damages or an injunction restraining further growth of the trees or requiring them to be reduced in size.
The legal position with respect to trees is unclear and taking action through the courts would be expensive.
In all cases it is best to negotiate with neighbours rather than litigate.
In some cases a neighbour may be able to make a complaint about a high hedge under the Anti-social Behaviour Act 2003.