Johnstones Trade Flexi Satin is a good option for UPVC window frames as it's flexible and durable. It's designed for exterior wood and metal, but it can also adhere to UPVC without the need for a primer, provided the surface is properly cleaned and lightly sanded for better adhesion. Many decorators use it with success on UPVC for a modern, refreshed look. Just make sure to follow the preparation instructions carefully for the best results.