It's a nonsense, and the CAA know it is. I sat in a meeting with FD at Gatwick about 3 years ago, and CAA didn't even realise that the restriction existed and agreed with us that it was silly and that if they could be taught in the aeroplane they should be able to be tested in it.
There are a lot of people who have happily ignored it and I'm quite certain that CAA have NEVER queried this, but if you are concerned, I'd recommend getting onto GAD at Aviation House and getting a letter agreeing that if you can teach in it, you can examine. Bob Commander is probably the fellow to speak to.
G