In message , Lawrence
writes

so to say,
if i'm going to The Royal Observatory from Waterloo,
i'll have to take the Jubilee Line till Canary Wharf
then transfer to DLR and carry on towards Cutty Sark where i'll alight?

Everyone else has suggested you view the Meridian at the old observatory
in Greenwich - and that is certainly the classic place.

But there are numerous other places in Greenwich where it is marked:

http://www.london-footprints.co.uk/artmeridian.htm

In particular, Meridian Point next to the Millenium Dome is only a short
walk away from North Greenwich on the Jubilee line. Whether you find
this as romantic a place as the old observatory, a couple of miles away,
is another matter - but it is at least somewhere where you can easily
have a foot in both hemispheres.

--
Paul Terry