This will of course have a negative effect on the BER rating for your house! Someone not far from us is building a new house. Built to current specs. Because he has chosen to have windows that can be opened he was told he will get a lower BER rating than if he installed non opening windows. I presume the powers that be that come up with these regulations do not realise that if you have a door fitted this will be opened and closed numerous times daily. Me, I like fresh air!
Regarding the catflap. If you installed another opening on a panel a couple of feet away from the first one, it would close before the second flap was opened by the cat. Might keep the BER man happy.