Hello enzyme76,
Thats a really good question. I can't comment on the practices for the U.K., because I am not familiar with the legislation.
In Canada, all pressure vessels and lines are required to be tested. This includes vacuum lines. There is a chance that a vacuum can be lost, and therefore pressure can cause the contents to leak.
I would ask someone within the company, because, there has got to be a good answer for your question.
Sorry that I couldn't help more.
Sincerely,
Eugene