I'm always impressed by friends who go to St. John for the first time and order the bone marrow. My dad always said when I was little (and still says, actually), "If you go to a hamburger joint, order a hamburger." Well if you could characterise St. John by one dish it would be the roasted bone marrow on toast with parsley salad, and that's the thing to order there. It's always on the menu and in the past 13 years it has become a classic (or rather, it's been rediscovered as a classic among the London restaurant crowd.)
Bone marrow and offal in general have had some tricky years this century. Mad Cow Disease didn't help (and we shouldn't forget that beef bones were banned in the UK for some time), but I suppose it all started after the war, when people started to be able to afford to eat more meat. They stopped buying and enjoying cheaper cuts and innards which subsequently fell out of vogue, as did British cooking in general, especially in the restaurant scene. Fergus Henderson of St. John is largely credited (and I believe rightly so) with the renewed respect for British cooking, and for offal in particular.
I'm still not a real convert, so I'm not going to pretend that every time I go to St. John I have the bone marrow, or liver or tripe or any such thing. Most often I have equally delicious but less "adventurous" options. It actually took me a year of almost weekly visits to try the bone marrow, but I advise you to do as I say, not as I do. The jelly-ish interior of the veal bones has a deeply savoury flavour perfectly complemented in taste and texture by the crispy toast and fresh parsley salad. If you can't get yourself to Smithfield (or St. John Bread and Wine in Spitalfields) then check out the recipe - brought to you by one minimalist from another.