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.