As I said,
VFR flight plans are rarely looked at, never mind checked for validity. But the scenario where a VFR pilot might get stuck is if he is getting a notam briefing for a route. The
NATS website mostly used in the UK enforces the correct ICAO flight plan format and would object to a route containing ICAO airport designators. Which leads to the Q: how do you get notams? I suppose a site
like this would not enforce anything.
Yep, I had that very problem. It took me a while to figure out it was the airfields it didnt like. I get around it by typing the airfield ICAO identifiers into the many alternate text boxes down the bottom of the page. That works for me.