What would be the advantages of making the London Zone Class D?
In relation to the helilanes...it would allow for some tolerance mentioned by the original poster and not mean an automatic loss of separation against Heathrow traffic , if the helicopter deviated slightly from the route (for example H3 as AlanM pointed out).
I am not advocating a free for all...and pilots would be expected to stick within published limits on the routes. I just think it would allow ATC and pilots greater flexibility to get into and out of central London.