I doubt you will notice any speed difference going from 32 to 28 on the same tyre variant. Likewise with puncture resistance and grip. The only thing you probably will notice is a loss in ride comfort. 32 to 28 is quite a significant decrease in air volume. What's your motivation for trying narrower tyres? Given the priorities you listed (especially comfort) I would stick with 32c. But if it's a supply issue then 28c are not going to feel that much different. I wouldn't bother with 25c on mixed uk roads. Those will just make your bike feel less compliant on the rougher road surfaces for no obvious gain in speed. Indeed they could well be slower.