Wow what a response, certainly caused a stir - not really intended, but i think we might have to agree to disagree on this one in terms of what is considered an actionable nuisance. Obviously if someone goes to court and pleads to case then a court could make a ruling, possibly in injunction, but that is a slow and probably very expensive process.
Some things that I can certainly agree is that in theory you might be able to demonstrate that light has caused so much harm to you or your property to get it claimed as a nuisance that you could then action (please note i did say harm or damage and that it can always be possible to go to the ends of the world to demonstrate harm if it is not physical damage). I would be impressed to see that case when it does come to court, but it is certainly not impossible.
The 2 cases you quote, the first relates to Japanese Knotweed encroaching on to a neighbouring property from Network Rail land. The issue about not causing harm, is a bit interesting given that Network Rail had allowed an awful lot of it to trespass, which in itself is an offence, but also it threatens property in both resale (which i think this was - without reading the judgement again?) and reasonable enjoyment as it grows rather aggressively underground and through surfacing. I bet the landowner got really frustrated with the lack of action by Network Rail and the ruling appears to be a good one with sound logic?
I do believe that the second case refers to light loss caused by a new building (please accept my apologies if not) and to my knowledge has never been upheld in respect of trees blocking light to buildings? My recollection of that case is also that the appellant had to demonstrate an awful lots of years of enjoyment of light to a particular aspect of his building.
I certainly would never advise anyone to use the actionable nuisance exemption to prune overhanging branches of a protected tree (TPO or Conservation Area) in terms of light loss, and would be concerned if any Arb professional did as it would be difficult to defend when a prosecution beckoned. But i suppose everyone has to give advice they feel is appropriate.
Having quite a few years of LA experience including a fair number of prosecutions & TPO appeals i would suggest that there hasn't been anything to lead me to believe that actionable nuisance in terms of light loss would apply to protected trees - that can obviously change.
Anyway lets see what happens to tree in question, and lets keep it nice going forward as these forums are really useful and it did make me read Mynors and look at the cases quoted again - every day we should all try and learn something.