UK ISPs could offer to operate a Mozilla TRR DoH server for two reasons:

1. The deal major "as seen on TV" ISPs struck was that they would offer a configurable child protection style filtering to their users. Mozilla permits users to opt in to filtering, you just aren't allowed to filter by default, so an ISP provided DoH server which can be configured explicitly to filter would meet this requirement. NextDNS offers this, yet is in Mozilla's programme. If you just pick NextDNS from the drop-down in Firefox you get no filtering, if you sign up and pay them (or take the free offer) you get filtering of your choice, and DoH, with instructions on how to tell your Firefox about this (basically paste a per-user URL into a preferences window, the nice thing about DoH compared to plain DNS or even DoT is that in a URL your user identifier will be encrypted, improving privacy)

2. The government did not legislate a requirement. They've been burned before on the difference between public appeals to think of the children (generally broadly accepted by the populace, no legal fallout) and censorship laws (likely to be destroyed in the courts because it turns out people don't like being told what to read). All those famous ISPs chose to voluntarily censor the Internet (mustn't let kids see porn) and then since they had the capability to censor courts told them to also obey Hollywood's instructions (no Pirate Bay either).

A small ISP like Andrews & Arnold isn't censored. During sign-up it says "Do you need child friendly filtering as part of this product?" or something. If you click "Yes" it says sorry they don't want you as a customer, good bye and the sign-up process is over.

I chose "oblige" carefully. Basically the ISPs AIUI/IIRC were given the option to voluntarily abide by a blocklist of domains - like thepiratebay - or have legislation made to force them to comply.

I'd heard A&A were an outlier here but didn't know they actively stopped users from choosing their service if they want filtering. That seems weirdly fascist: like a supermarket that won't let you choose not to have Coca Cola on your shopping list, if you don't want it you have to actively remove it yourself.

Porn, yes to some extent, but super-violence, torture, malware, gambling, prostitution, ... these are all things I choose to attempt not to pipe in to my home via OpenDNS/pihole/uBlock/direct instruction to local users!

I don't agree that choosing not to offer a product or service which is popular but which you believe is a terrible idea is "weirdly fascist".

I would suggest instead that demanding other people figure out whatever weird quirks you have and then cater to them under the guise of being "child friendly" is at best weirdly fascist.

And it seems you at least reluctantly agree this that can't work even if you wish A&A would try to do it anyway, since all the systems you listed involve you explicitly configuring what you want blocked, allowing you to take responsibility for the inevitable under/overblocking. This approach works fine† with A&A since it doesn't ask anything of them.

† Well, as "fine" as can be expected, no worse than at other providers.