That is a good question. I am not familiar with the laws in the UK , but most of the UK is more than 400' above sea level, so I would ASSUME that is is 400' above ground level.
@holtneil lives in the UK and can probably give you a better answer.
Yes my understanding is that it's 400 above sea level. The other stupid question would be how do I find out if where I live is above and by how much lol
@holtneil lives in the UK and can probably give you a better answer.