http://www.leics.gov.uk/glazing.pdf

for example. (You need Dopey acrobat)

Thanks for your trouble finding that. I did in fact find it on my travels,
and understand I need safety glass.

The Question is: how thick?

There appears to be no safety rule regarding thickness, provided you use
safety glass of some sort.
Will it break under its own weight if I use 4mm toughened glass in a single
door-sized sheet?
Will it break under its own weight if I use 4mm toughened glass in
half-door-width sheets?

I know I can buy 4mm toughened glass, bot plain and patterned.
I suspect I cannot buy 4mm laminated because it would be hard to produce? Is
this true?