@Teefteefteef I do sympathise massively, and this exact same thing has just happened to my friend's sister who is 30. She had a filling come out too (on Christmas Eve, typical!) and when she rang her regular NHS dentist on 28th December, she was told she had been struck off the list. She was really upset.
She complained 'yeah but Covid' and so on, but like your husband, she had not been for 3 years. (Last time Nov 2018!) Covid didn't start til March 2020, so that's not a great excuse really... She was meant to have gone for a check up in May 2019, (6 months after her last one,) and cancelled it because she couldn't be arsed to go, and didn't need to.
The dentist was closed because of covid from March 2020, but reopened in late July 2020 until November 2020. So she had a full 5 months to rebook her check up. But she didn't.
They said they wrote to her, (May 2021,) and gave her 3 weeks to contact them, or they would de-register her. But she moved house in October 2019, and never gave the dentist her forwarding address, so it's not really their fault.
She complained that they could have rung her, as she has had the same mobile phone number since 2015. But as has been said, they are a business, and will drop non-attending/poor-attending patients for regular ones, at the drop of a hat. So, whilst it is shit for this to happen, this young woman, and your husband, need to take responsibility themselves for being dropped.
It is common courtesy to let someone know they are about to be struck off, but as other posters have said, they don't have to. It's not law. It sucks that this happens, as you wouldn't be struck off the GP list for not attending for 3 years, nor would it happen at the opticians, but everyone knows what dentists are like, they are really stringent, and you do have to keep up the visits.
Hope he manages to get re-registered with another NHS dentist @Teefteefteef but I doubt he will anytime soon.
Good news though, this young woman I know has an appointment at a private dentist on 7th January, and it's only just over 50% more than the NHS. So the NHS price would be £65 for the treatment, and new private dentist is £103. The only time there would be an issue/huge costs would be if she needed crowns, or bridges.