The selection of springs on webites such as autodoc/ autodeler.co.uk are all OEM equivalents not upgraded or sports springs so there should be no need to inform an insurer.
A non OE spring dimention is something else.
Maybe OP, Gavinm , should take a look at another similar sportage and if you are satisfied with the ride height of that, then just replace the springs with stand ones.
I have taken a quick look at the art nrs
553503U100 and
553502Y100 which may be the correct nrs for this sportage, only a few of the springs are listed with the length and wire diameters, and the variation is minimal, only about 6mm in length and 0,5mm in wire diameter. I had expected a greater variation.
I put KYB K-flex springs on the front of a car a while back and the ride height ended up higher even tho the spring length was equall to the factory original. The K-flex had about 0.75 coils less than the original spring.
So to sum it up dont buy the Monroe springs, they have the thinest wire and may be softer. Mapco have a slightly thicker 12mm wire, 3mm larger diameter and may be just a shade stiffer.
The others brands appear to be all the same, 334mm x 11.75mm dia.
As far as the shock absorbers go, I would change them regardless just to avoid having to work on the car again. If you are intending to keep the car more than a couple of years you'll probably be happier knowing both springs and shocks are new. Should be 15-30 minutes each depending on how rusted the bolts are.
I have never been disapointed by a KYB product.