I reckon the quality has gone down.
Back in my 80s childhood, I had a definite hierarchy of chocolate:
Lindt
Suchard
Thorntons
Frys
Cadburys / Rowntree
Thorntons was (to my mind) much closer to Suchard in quality than Frys. Also its chocolates had a definite flavour unlike for example Roses or Quality Street, which to my mind taste the same now as they did then (ie, a sort of generic "sweet" flavour).
I left the UK for NZ in 2001. Thorntons was nice then.
Visited in 2011 and bought some from one of their shops to take back for the kids. I don't remember anything being wrong with it, but I might not have had some.
Saw some recently in the local supermarket for not a very cheap price. Bought it. By golly it was disappointing. It had the same generic "sweet" taste that I associate with Quality Street etc. Really no better than Cadburys (which to my mind has also got worse) and considerably worse than the local brand Whittakers which I do recommend as a perfectly decent non-special-occasion chocolate. No way would I get it again at even half the price I paid.
I will add that the standard of chocolate in the UK has definitely gone right up since I left
and the value has improved. It wouldn't surprise me at all if Thorntons had tried to reposition themselves a bit further down the quality rankings. And finally, be grateful for what you have. You have to sell a kidney to afford decent chocolate in NZ.