Reason? I think it's because I was 41 when I started.
The implication of your post is that the hiring managers responsible for 3500 jobs are *all" discriminating on age. That seems unlikely, if only because there are people in the UK tech sector older than that who can still find roles, so evidently not all managers discriminate.
It is endemic in the Anglophone world. I also speak a little German, French, Norwegian and at a push Spanish and Swedish, and I applied to anywhere that was hiring English speakers there, too... with cover letter in the local language, naturally.
I think it's just an oversupply of people in it support and it really contrast with the shortage of software engineers. Now that you're teaching English as a foreign language you're in a skillset area with a shortage and that's why you're more in demand. Age might matter a little bit but I think you've just got the right skill set now in the area that's short of experts.
The implication of your post is that the hiring managers responsible for 3500 jobs are *all" discriminating on age. That seems unlikely, if only because there are people in the UK tech sector older than that who can still find roles, so evidently not all managers discriminate.