After PanAm 102 i became policy at LHR that all pax and bags leaving LHR, either in transit or originating, would be screened by Heathrow security.
The 1 bag policy is current DfT directive which is hopefully under review.
The T3 transfers facility is a disgrace, but is soon to be replaced by a new facility directly behind the current one with 7 x ray machines.
Complaining to security officers or supervisors, or on pprune, will not get things changed - they agree with you 100% and make their feelings known to management regularly.
All frontline staff in T3 would urge you to complain strongly to BAA via the website
www.heathrowairport.com/feedback and make it clear that you intend to connect in AMS FRA or CDG in future, and that you will adjust your company's travale policy to reflect the same. BAA don't fear much, and sit in an enviable position of a queue of airlines waiting to use the cramped facilities. The main thing BAA fear is being overtaken by a rival airport on on the continent.
Complain, complain, complain.
Also, Canadian security, just like UK security, is only as good as the guard sitting at that moment, looking at that particular bag. Screen reading is incredibly boring, but possibly one of the most important parts of aviation security. Hats off to all that do it.