IANAL and legalese in english is worse than in german to me. UK may give out warrants with gag orders. In some cases these warrants might encompass services "done" abroad, in other cases they have to go through other countries govs as specced by treaties, as specced by section 79. Correct so far?
What about a british subject, personal or corporate, working on foreign soil for a non-UK company. Could he be served with a warrant with accompanying gag order to retain certain data at his foreign place of work, aka drafting him into intelligence operation via warrant?