It’s cheap for provisional because you’ve got a experienced driver in the car with you, teaching you.
£1.8k is cheap. At 25, I was paying £6k to insure my RX8. It was £467/month with a massive first payment of nearly a grand.
I was paying over 3k to insure my ford Ka when I passed my test.
I’m 29 with a clean licence and it’s still over a grand.
The postcode is the main factor, mine is the highest risk.
The third party vs fully comp thing isn’t that fully comp isn’t much more, it’s that 3rd party is expensive. If you’re not arsed about your own car being covered, you’re likely not going to be so careful. I.e if you break it, you’d buy another banger.
The value of your car is irrelevant. They’re concerned the insured party is going to crash into a brand new S Class, or several cars.
Honestly, the cheapest way to be young on the road is to find a just-add-fuel deal that accepts that age, and do that. Most only accept drivers aged 21 up, but my nephew has a brand new fiesta,he’s 19 and passed his test last year. It’s about £220/month iirc.