This really does seem like an ill conceived solution to a problem that doesn't exist - or hasn't existed. The majority of pilots getting clearances through the London airport CTRs are professional rotary who get a good slick service and clearances from ATC - I hope others would generally agree. On the odd occasion it is clearly really busy we try to avoid if we can. And no doubt the odd SE fixed wing PPLs seeking transits do that even more.
Any review of the RT from a CTR over an hour or so will surely reveal that unless ATC is getting info in real time from aircraft, the integration of transit traffic cannot possibly be optimised or efficient. Even 2 mins in a 109 from a zone boundary is 5 miles! So why bother with the admin of what is almost bound to be a very inaccurate flight plan? The overall workload for all involved - pilots, ops depts and ATC will be vastly more for no gain. We live in an age where increasingly people think just because an electronic system can be set up, it should be used. Wrong.
But also let's have more consideration for the value of the time of the many pilots who would have to end up filing these pre-notification flight plans, as Duchess said. It takes 10 seconds to request a clearance over the RT. And I bet it will be 5 mins - 30 times as long - to file on line. And by the time ATC make the inevitable comments about the pre notified routing request I expect overall RT time will actually be longer.
And I can guarantee too that if mandatory this would decrease safety by adding to pilot stress when he has to refile when pax are early/late/change route etc before he can lift. Or as someone else said, he will be pressured to fiddle and file en route. Either adds to the risk of mistakes being made.
The traffic seeking this transits has not increased much I'm sure. As Thud says, if it's come from the E strip change, and that is now complex with 3 sub menus to dip into, focus should return to the programming of that to make it as quick as jotting down by hand.
Anyway hopefully this 6 month trial will prove that it doesn't work and is unnecessary.