Having looked at most air dropped training and their practice areas, I did notice I found almost nothing on mining. As this was an important part of operations in the early war years, I assumed this was merely carried out by using practice bombs on the local range but as it was ultimately over water did they practice specifically for this over sea and if so at what height?

I did come across canal versions and with definite use of the Foxcote range canal target in Buckinghamshire (possibly also Coombe Hill?) and also the use of various drains in the Fens.