"...but clearly UEA was then seen as a perfectly respectable choice for a potential Oxbridger."
"It wasn't a thing when I was applying back in the 80s - there was something of a pecking order but it seemed much less rigid than now."
I very much agree. Like you, I was applying in the early 1980s.
I can't claim to have ever been a potential Oxbridge person, but certainly when I applied to universities there was no apparent differentiation between RG and non-RG.
The real differentiation back then was between university and poly (nowadays post-92 universities).
Your post prompted me to think back to the universities I applied to - one of them was UEA.
The list was:
UCL
Warwick
Exeter
Lancaster
UEA