@landy-lee
If you ALTER the chassis, which you would prolly have to do to fit the springs you want then it becomes a radically altered vehicle.
So read the regs.

(This from the govt website

"Get a ‘Q’ registration number
You will not be able to keep your vehicle’s original registration number if one of the following applies:
- it has fewer than 8 points
- it has a second-hand or altered chassis, monocoque bodyshell or frame
- there’s evidence that 2 vehicles have been welded together to form one (ie ‘cut and shut’)
Your vehicle must pass the relevant
type approval test to get a ‘Q’ prefix registration number."
How to register your vehicle and the cost - new registrations, kit cars, rebuilds, radically altered vehicles, old and classic vehicles, vehicle identification numbers, Q plates
www.gov.uk
So the short answer is, yes, you can do it legally, but it is a bit of a faff. The test for type approval may or may not be a pain.
IVA shouldn't be neccesary I doubt, just a visual inspection, which is quicker and cheaper.
Once it is properly registered then insurance, even if you have to do it through a specialist, isn't that difficult and can be surprisingly cheap.
This from a bloke (me) who rana kit car club for years and has known about SVA and IVA for a very long time.
There are of course those who do it and don't tell anyone, the vehicle will pass the MOT although the tester may cock an eyebrow or two, if you don't tell your insurer, fair enough you take that risk. But the chassis wil or should have a number on it. Something else to be considered.
Speaking personally I'd either not bother or do it the proper way, just make sure you have time on your hands and another vehicle to run around in.
And there is this old thread on doing it here.
Hi has anyone here done a series 2 or 3 with coil suspension on the original chassis ?
www.landyzone.co.uk