According to .gov.uk:
"Operational low flying is an essential skill and is practised by fixed wing aircraft between 100 feet and 250 feet; a more representative altitude at which pilots would fly in a combat scenario. The final decision to use a TTA is taken on the day itself as this kind of training can only take place when weather conditions allow.
When a TTA is active, routine low flying by other military users can take place between 500 feet and 2000 feet. When the TTA is not in use standard low flying training is permitted down to 250 feet and helicopters down to ground level. Late spring and summer are the busiest times for low flying as squadrons make full use of good weather to carry out their training."
There are films on YT showing low flying fast movers at dusk, so I should stay away. And don’t forget, choppers can go all the way down to Surface level.