The description was "barely 50 ft above the top of the trees" so probably 100ft+ above the ground. Taking into account low flying planes always look much lower than they really are I'd still say that is much lower than it should be
Just so that you are all aware, crews are authorized to an MSD - a minimum separation distance. Fixed wing ac are routinely cleared to 250' MSD, and exceptionally to 100' MSD when trained to do so and when in the relevant 'Tango'. Helicopters (not my area of expertise) can be authentic down to 10' (I think). My point is twofold:
1, MSD implies a bubble around the ac, '50 above the top of the trees' means this bubble has been infringed and is therefore an issue.
2, The correct way to report an alleged breech of the UK Low Flying Rules is not on a public forum.
I expect the original poster is not a low flying NIMBY, but actually something of a fan of low flying ac, this raises further points that might be considered by a similar 'fan' before posting:
1, It is exceptionally difficult to judge the height of an ac above the ground. I struggle and I would consider myself an expert at judging 250'. It is fair to say that you are not experts so don't try. I had someone in the lakes file against me. They considered themselves experts (they occasionally frequented the M6 pass at Tebay to view the jets so, 'knew what they were talking about') and stated that I must be below 200'. My HUD video confirmed that I was at 406' at the time of the incident!
2, If you are a 'fan' and want some kudos by posting a 'I saw a really neat ..... ' thread, then just have an awareness of the rules and your potential audience. Journos will pounce on a hint of impropriety, and there are more than enough we-hate-the-military on this forum and others.
This may sound like a storm in a tea-cup, but I'm sure we all remember how much trouble a well natured GR4 Nav with a sign got in when a well natured spotter posted his picture on a well natured forum. All I ask is that you think.
Cheesy