T Cut do a special headlamp restorer kit, that is OK on plastic. ordinary T Cut is not recomended.
I do not know what the difference is.
Whatever I have tried they seem to go yellow again after a few weeks.

Has anyone got a solution that keeps them clear longer?

I used T Cut, and then Fast Glass from Autoglym,
T Cut is not recomended for plastic lenses but Fast Glass is,
I found the only way to clear the lense was with the T Cut though,
The fast Glass removes any harmfull chemicals contained in T Cut.
Fast Glass on its own should maitain the plastic ...

As for Different size wheels and yet maintaining correct speedo callibration.

There are tyre and wheel calculators online which will tell you what combination of wheel size and tyre profile you need to maintain your correct speedo readings.
For me it was more difficult due to wanting run flat tyres ...

Yes they are not cheap,

I have the original set plus spare 4 sale if you want cheap £100

Also you will need a tyre pressure monitor installed, this will tell you pressure and temp for each tyre in real time.
Otherwise you might not realize you have a puncture in the first place.

The size is ...

Yes it is true I have run flat tyres on my V6 2wheel drive Bongo

They are Cinturato P7 all season 225/50 R18 99V On stunning white rims.
The handling and ride are excellent, (thought a little too soft before)

So no worries in regards to a puncture as can still drive 100 miles at 50mph and no need ...