UK considerations: must be at least signed off by an approved electrician ("Part P" regulations), and for any situations involving subsidy needs to be MCS approved as well. https://mcscertified.com/
Find an electrician who will inspect and sign off on your work. It's not illegal as licensed master electricians have mechanics do all the grunt work who are unlicensed. The electrician vouches for his employees work which legitimizes it. No different than you doing the work and having the electrician inspect, call out issues to fix, and inspect again until no issues are found and sign off.
Surely it only needs to be signed off if you intend to sell the property with them or sell excess back to the grid. If youre just using the batteries how is anyone going to know?
If the DIY work wasn't the cause for the fire it shouldn't matter, but I half-expect someone to inform me that US insurance companies can (legally) deny coverage for reasons unrelated to the accident.
Not so fast. Have you very carefully read the full small print of the insurance policy? Did you review that with a lawyer? Is incredible how different "normal" people vs. lawyers can understand a contract.
I'm pretty sure there is a clause, which states that you have to inform if you have and/or are not allowed to have fire loads, or anything that could cause a fire, or make it worse, or something along the lines in legalese. These formulations are always there because of people hoarding fuel in the basement, for example, or O2 Tank, or whatever. They are formulated in the most generic way possible to catch anything you do "wrong". Failing to follow such clauses, also when not explicitly stated, is dropping your obligations in the contract. And then there will be a clause that of course says, that not following the contract from your side, also exempts the company of paying.
Note also there are clauses that are very softly specified, like "use rooms for the intended purpose" which may be a problem if you store idk, paint in the garage, which may be flammable, in which case a fire in the garage will not be (at least fully) covered.
If is something does happen, and Li Batteries catching fire is not something unheard of, you will be in a world of suffering. Probably having no home, and having to pay damages to the neighbors. All to save what? 2k$... 5k$?
Theoretically a good choice, but where I live, just doesn't work. Either they just say "no thanks" or they will be more expensive than letting them do the whole job.
I got the "trick" recommended to do the things yourself, then call a certified guy, and say "look, I contracted a guy, I had no idea, he came did everything, but I got a bad vibe, I would like you check the whole installation". But it also does not really work, they will come with a contract, where you are enforced to contract them to correct any findings. And boy they will find things then...
It "may" not be permitted, but if you live in a collection of shacks in rural Colorado that were themselves -already- completely un-permitted then you might decide that it's best to just do the work yourself.