Within the UK it is illegal to use staff search unless you are listed on the paperwork as operating or supernumerary crew, indeed at LHR if caught you will lose your airside pass.
As management at a previous airline we had discussions with dft about this very subject and it is an absolute no-no, we were told in no uncertain terms that positioning crew must be in possession of a valid boarding card and been through passenger security. Simple as.
Don't forget that any positioning crew on a boarding card (even in uniform) would be subject to airport taxes, maybe that's why Ryanair do it this way but I think they are playing with fire within the UK.