Any question answered...

Unfortunately I think you are just going to have to find the cash somehow. Insurance won't cover anything identified pre-policy.

It is an absolute crime the state don't do more to help with teeth and eyes.

I seriously recommend that you register with nhs dentists in Fulham, I've been going to them for about 8 years (had a nightmare trying to find any decent NHS dentists in London before that) and they're just brilliant, they're basically what dentistry on the NHS should be. They specialise in NHS service so all the fillings and root canals will be done at NHS band prices (unless you want white fillings and even then the price is lower that every where else ) and they wont try to up sell you. They offer private dentistry for things not covered like implants (I had mine done there) but if you can qualify for NHS treatment due to trauma they'll definitely tell you. I live in forest Gate so it's it's a bit of a trek across town for me to get there but it's definitely worth it. At the very least they'll be able sort out the emergency stuff for you without you having to sell a kidney https://www.nhsdentist.com/

Only thing is, I need serious (almost emergency) work done. Think 4 root canals finished (potentially 5), 3 implants, and plenty of fillings.

Not the best timing but you're better off going abroad.

A long time ago now, but as an eg it was significantly cheaper for my dad to fly with me to Barcelona and stay in a hotel and have an implant there.

From my Croatian colleague, Croatia also sounds like a good option, as does Slovenia.

These types of procedures require quite a few staff as well as the operating space, so the reality is that in the UK it is expensive and gives some idea of the real cost of medical procedures.