Re: Even a Portable Faraday Cage? @AC
OFCOM is the regulator, you need to read the Act. s55 is all about this:
55 Enforcement: use of apparatus
[OFCOM can do nasty things to you if, inter-alia]:
(a) the use of the apparatus is likely to cause undue interference with wireless telegraphy other than wireless telegraphy falling within subsection (2) [essentially police, fire, coastguard etc];
(b) the use of the apparatus in fact has caused, or is causing, such interference; and
(c)the case is one where OFCOM consider that all reasonable steps to minimise interference have been taken in relation to the wireless telegraphy station or wireless telegraphy apparatus receiving the telegraphy interfered with."
So, is it causing "undue interference" is the bit you need to consider, coupled to have reasonable steps been made to minimise its interference.
Also, whilst not explicit (there is no definition of apparatus I can see, but IANAL, I just looked at the Act) the Act is written with the idea of broadcasting/receiving in mind. I have no idea if it would treat a Faraday cage as "receiving" a signal, and therefore be subject to it in the first place. I doubt it, or every building in London would fall under its remit.