maybe a stupid question but Ill ask anyway. Just say I get insurance for a standard car, I can declare modifications once I've already taken out the insurance right? like a month or two after I got the car.

Also if I got the insurance and did say past plus in a month or two can the insurance go down if I'm paying monthly or would I have to have done the past plus before I take out insurance for it to go down?

yes to both questions. They'll probably charge you an Admin fee to change the policy part-way through though.

Yes but be careful as 90% of Uk insurers will not allow you to modify your car.. also you'd probably find it more cost effective to declare them at the quoation stage..