uPVC has to be fitted at the time of manufacture as it weakens the door if fitted after.
My door already had a catflap, I have the non magnetic Staywell, 4 way lock flap. When locked to in only, two of mine can still break out
When locked completely they do their damndest to get out and have succeeded once using brute force to prize the frame out of place a bit and pull the flap open. Problem with that is they couldn't get back in
So if I lock it I have to put a laundry basket in front of them so they know going out isn't an option but even then they'll sometimes shift it to have a pop, usually at 3am when it's guaranteed to annoy me most