I painted a couple of yellowed UPVC windows for a customer about three years ago, it still looks good and the customer is happy.
I used the Dulux UPVC paint.
There is a primer called ESP(easy Surface Prep) that prepares almost any surface for painting. That should work well. Plus, I am sure that there are alternatives to this if you ask at your local decorators centre, or even in a store like B&Q.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local