You've had it looked over by a good tech, so got an independent evaluation. That leaves high humidity as one culprit. Use a hygrometer to measure RH in the case and see where things really are. Didn't the UK have an unusually wet and hot summer? My guitars certainly sound better to me at 40% RH than at 60% RH. Some guitars are just more sensitive than others.
The other possibility is something that I really don't believe in - the guitars just "went to sleep". Get some aggressive play hours on them and see if that helps.
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