Hugh, the brake discs are Girling items, you will find the part numbers in parts list in the Resources section on this site
here .
This parts list is for 409/410 but when I bought front discs for my 411 they had the same part number. I can't guarantee that they are the same as the Britannia but it's very likely they are.
We know Jag wheels (XJS) fit a 411 and a 603, so the stud pattern is the same. That's a start.
I have a set of new front discs in my garage and I can easily measure the disc diameter, thickness etc if you need it.
Kevin