No idea, haven’t heard anything about anyone selling lenses or lens sprays ove the web.
Being honest - headlamp reflector can be split int two sections. That makes renovation even easier. Only thing to go for - is not a spray, but vacuum deposition of aluminium.
And yes, the glass lid simply detached while wobbling against the headlamp bezel. Remaining headlamp was in similar condition (stronger pull and off we go).
And yes, butyl rubber was used from the outside to seal both sections better:
I prefer term “diversed”.
There is however one common thing with XJS owners and UK. Imagine yourself women overdosed with adhesive plastic elements and makeup that according to tribe knowledge, when joined - are meant to make them more attractive. Almost in the same way like people packing their XJS interiors with gadgets and addtions to the perfection, ending up with christmas tree instead of cockpit…
We had a whole thread on that subject … Guess you missed it … The vendor is taking deposits down on new ones prior to running off a batch of them … I think it’s $300 per side … ? 
Just tried search option and haven’t found anything about British women being augmented with plastic composites (please advise)
Quite expensive in US, got £40-60 per UK headlamp in EU. The trick is - you need to prepare it (remove glass and old glue)