Insurance of the first car

Hi guys,

I'm going to buy a car in April/May (I would be 21 then) but the problem is that I've come to the UK in September last year cause I'm from Poland.
That would be my first car here.

Ive checked everything about the insurance, but I would have to pay more than £1200 even for a Corsa C 1.0... why?
Any propositions about the ideal first car with cheap insurance? (I hate diesels...)
I've got £4000 for now, so I can spend max £2500-3000 for a car and the rest would be insurance, oil change etc.

Any ideas?