Owning your own home

Oof that is shit.
I'm no expert but worth finding out if they have a certificate of lawfulness for the work.
And they are still subject to planning laws even if the work is broadly allowed under the permitted development system

You need to contact your local councillors, not council officers. Councillors have a more direct line to all the services, whether that's police, the fire service, environmental enforcement, planning, and so on. As Dugtheslug says, keep a detailed diary of everything that happens. Neither you nor any of your neighbours should be involved directly. The services need to do the work for you and you shouldn't be exposed. What you have here is anti-social behaviour, no doubt multiple planning breaches, probably HSE-notifiable safety issues, as hoefla says, and the drugs issue and the sort of veiled threats being made suggest that the police should already know about this group. If no action is taken now, it will only get worse, so don't think it'll settle down if you leave it. Good luck. But yes, contact your councillors asap.

I’m not sure you could make this up!
Bastard neighbours thread material. I can’t offer any advice apart from moving.

I’m really sorry to read all of this. They sound like actual criminals.

Did you mention you had noticed them sell drugs to kids, from one of those cars and from the front door?