In article ,
says...
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?

4mm toughened glass is perfectly acceptable for use in your door.
It is the most commonly used thickness.
4mm laminated (nominally 4.1mm) is a bit thin for a single sheet in a
door. (An area of heavy traffic)

The "Safety" designation is not that toughened glass will not break but
that, IF it does, it will break into tiny pieces which will not seriously
cut rather than into dangerous shards
--
Ziggur

"S'ils te mordent, mords-les"