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I'm not so sure, my understanding of highway code instructions and whatnot is that other roadusers(cyclists included) should give space to cyclists as they could stop/slow/turn without warning, some riders may not have the skill to signal or the surface they are riding on prevents them from doing so as they normally would. Similar to driving really if you tailgate someone and they stop it's the driver behinds fault. It's still more courteous to signal but it's DfT documents so everything has to fit in with everything so can't suddenly make it compulsory, even in 4.1.1 they mention "the risks associated with giving arm signals, such as reduced stability".
that was my thinking on the matter, it comes up later but should be there from the get go!