You can actually get a traffic service from anyone with a Radar, for example I have got TS from Doncaster who eventually handed me off to Waddington. Some are more helpful that others, for example Solent never give a TS, though may give traffic info on a Basic service if pertinent (i.e. you're about to hit someone). I suppose it is that they don't want to start or else they will be doing it forever.