That's rather a nice fossil! As far as I know, Titanites
is the largest ammonite known from the UK and examples are also known from areas of Northern France. It is an upper Jurassic form and examples have been found up to 2 feet in diameter. Most of these larger ammonite forms seem to date from the Upper Jurassic or Lower Cretaceous.
Britain loses out to America in ammonite sizes (of course!); the North American ammonite Parapuzosia bradyi
is known in sizes ranging up to 4.5 feet in diameter. Still larger was the Cretaceous period Pachydiscus seppenradensis
from Germany which could attain a diameter of 6.5 feet, a truly monstrous beast.
Here's a link to an incredible example of Pachydiscus
(scroll down a bit, you really can't miss it):
One really has to wonder about the lifestyle of these enormous ammonites. They would have been unable to swim very fast due to the sheer bulk of their shells and the unstreamlined shape they adopted. It also seems unlikely that they would have had ink sacs; there would be little advantage to be gained of producing a phantom image of itself in ink if the creature could only slowly move away.
I would imagine that the adult forms of these large ammonites would have been deeper water creatures as it seems that a stronger and larger shell may have been able to withstand stronger water pressures. Perhaps they lived during the day at a depth below the maximum diving depth of the marine reptiles and probably vertically migrating to shallower depths at night. Otherwise I can imagine they would be slow moving fodder at shallow depths where they could be easily spotted. Even a form such as Titanites
would probably represent little trouble for a pliosaur.
I'm sure studies have been undertaken looking at the associated fauna found in the same deposits with these large species; it would be interesting to determine the depths they are thought to dwell. It would be also be interesting to know if there is any evidence of marine predation in these forms.