I can't comment about your insurer or what SEAT stipulate are 'allowable' wheel and tyre combos (I suspect its more what they will fit with their stock of OEM alloys vs the minimum and maximum alloy diameter for the brakes and steering/suspension), however:

I changed my Mazda3's OEM 16 alloys a few years ago for similarish reasons (the ride previously wasn' that harsh, but I drove a near identical car with 15in wheels and higher profile tyres and the only difference was a better ride) as well as needing to change one anyway that was corroded and causing a tyre leak (the others going in a similar direction after 12 years).

I spoke to my insurer at the time (esure) what, if anything I would need to do if I was 'downsizing' (same rolling diamter within the +/- % limit) from 205/55 R16 to 195/65 R15 and they said 'just tell us', but I also asked for a requote, as I deliberately wanted to get the work done as my car was due to be re-insured anyway.

There appeared to be no change in premium for this downsizing, noting that the size combo chosen was one of the three 'allowable' ones on the car's door plate and handbook (the other was upsizing to 17in). The actual premium came in lower than the previous year (same conditions otherwise), but that wasn't attributable to the downsizing, probably just market conditions at the time (March 2018).

I suspect if had upsized, it would've been treated as a (negative) 'modification' in terms (for them anyway) of safety and been charged a bit more in the premium, though I didn't check.

Going outside of the 'allowable' alloy and tyre size combo might attract a far larger bump in premium because (for example) the tyre is much bigger/higher performance and/or rarer and might indicate you are a faster, more risk-taking driver, never mind the extra expense of both wheel and tyre if needed to be replaced after an accident.

Whether going a non-OEM route (even on the allowable wheel size - don't forget it's also what width of tyre each accepts) makes a difference as well, I don't know - possibly.

If you're downsizing, I suspect like mine it'll be cost neutral if you do so when the car is due to be re-insured. Outside of that time, you might incur an admin fee of (say) £15 - £30 for 'changing the policy'. For some (better) insurers, they may waive this for minor changes like this, especially if you're a long-standing customer. Ask them to see what you can get - that will likely mean telephoning.

Whatever the case, make doubly sure the new smaller wheels fit over/don't touch the brakes!

Assuming the car is already insured, there is no reason a broker should be involved. Just get In touch with your current insurer and let them know.

As long as the wheels are the correct fitment there should be no issue or reason for the price to go up. The tyres, well as long as they are of the same circumference (or within a couple of percent anyway), in theory, there wouldn't be a problem. But to be sure, you should use the same size tyre as lower spec Toledo's would have came with anyway. On a 15" rim, I believe this would have been 185/60.

I drive a Toledo and have done exactly what you are looking to do. Few years ago when the tyres needed replacing I purchased a set of pre used alloys found being sold by a Skoda dealership (ie original VW spec alloys). The car came with a full size 15" spare so I knew this size would fit. I put CrossClimate tyres on the alloys and have the originals in my garage which I could swap back or sell on as needed at a future time.

The ride is better and the fuel economy might be better too (difficult to be 100% sure). Certainly I saved money with the cost of tyres, 15" tyres are considerably cheaper as befitting a smaller and a more popular wheel size. Of course, with an older suspension design there will be better riding cars out there, but much more expensive too.

I did the same thing with my previous car too, down from 17" to 16" and ran summer / winter tyres on the smaller size alloys. My insurer was pleased to hear from me but advised there no need to discuss regarding use of winter tyres and regarding a change down in size. However, a change up in size must be declared.

Edited by Smileyman on 07/02/2023 at 15:25

If you are selling your 17 inch alloys in great condition I may be interested. Dynamic I think. Or even one. I have quality all season tyres and can live with the harsher ride.

I bought my car off a Seat dealer Mechanic who does my servicing and fixes...two things to watch out for, rear wiper failing resulting in needing a new motor. Easy fix for the mechanically minded. Leaking coolant from housing so needed a new water pump and housing and the belt change was due, so something to look out for on a 6 year old and over. 50k and no other issues apart from it takes an age to defrost on icy mornings!