Dude~,
I almost never file my flight plans with the ATCO. If I'm departing from a farm strip there isn't one of course, but even when I'm going from a controller-equipped field I normally just fax the plan to Heathrow, often the day before the flight.
They have computers and faxes and clever things like that and propagate the plan to whoever needs to see it. After all, you don't fax your plan to your destination airfield, and yet they magically know about your arrival.
A web-based system would seem like a useful improvement, especially as it could validate a lot of the information as you entered it, thereby avoiding many of the errors which must crop up on a lot of flight plans at the moment.