I wouldn't be surprised if a lot of UK PPLs were not aware of the requirement to file a flight plan for controlled airspace. The problem is that in the UK this is not done through paper filing, and as such a lot of people won't link that booking out or talking to ATC can constituted a flight plan being filed in an abbreviated form.
Certainly if someone asked me in the UK "Do I need to file a flight plan to go from A > B through class D?" my answer would be no, because if I said yes they would go and file a full flight plan, which is not required.