Flightplans and TMAs
 

I was recently talking to an LTMA radar controller and they said that they do not have access to flight plans for aircraft transiting, unless it is airways. Surely this partially defeats the point of filing flight plans and actually complicates the process - increasing RT, preventing coordination down the line, more workload and more ambiguity?

Unless you specifically address a flight plan to an en-route unit, they will not be sent your flight plan. This applies both to ATC in CTAs/CTRs and to LARS units in Class G airspace. As far as I'm aware, only London and Scottish Information have a system to 'interrogate' the AFTN to get your FPL.
You file a flight plan so that your destination knows when you are on the way, calculates your eta from the FPL details and can take overdue action if you don't arrive within 30 min of that ETA.

Sure. But wouldn't it make sense for busy enroute units to have access to your planned routing? It is perfectly, technically possible.
NATS and the CAA complain about busy airspace and yet they don't take simple steps, to join up the dots and make it easier for the controllers.

These 'busy' en-route units are, well busy enough with their own traffic and don't 'need' extra distractions, especially where they use a system like 'Copperchase' which automatically prints flight progress strips from flight plans it receives but will disregard flight plans for traffic not inbound to or originating from them.