The Stressless are that price because in 20 years, it will look much the same as the day you buy it. The others will not.
There are some really shonkey quality chairs being shipped in from Vietnam and other similar places just now. They look ok and will be £399 or less. Fine for a few years but not much longer.
I’ve been in the furniture trade for 20 years, bought a Stressless chair in year 1 and am sat on it right now.
It’s one of those things that’s worth spending the extra money on.
I would also suggest going to your local INdependant rather than a big group, to buy it. They Often sell a better version.
ETA - your idea to buy a 2nd hand one is a good one. If you can find a nice one that’s had little use, that’d be great!