change upvc door panel for cat flap?

There was a very helpful explanation on how to change a upvc door panel. I am looking at possibly trying to put a cat flap into the white upvc panel in the door. However it is in a rented property and the landlord has agreed so long as I can return the door panel to one without the cat flap.

Do you know where I might be able to buy (UK) a spare / 2nd plain white upvc door panel (low cost, of course) so I could follow your instructions above on replacing them? Also any advice on fitting / cutting the upvc panel for the cat flap greatly appreciated.

Thanks,
Simon