I'm not aware of any other UK glass suppliers to new vehicles built in the UK in that era. All MG's had triplex as far as I know, and I think that Triplex had cornered the market.

I assume you know about the year of manufacture dot under the word 'toughened'?

This is an interesting link which leads on to other interesting links!
http://www.gracesguide.co.uk/Triplex_Safety_Glass_Co

And this one about MG's:
http://www.mg-tabc.org/library/triplex_glass.pd

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4