I suspect that the reason you haven’t had an answer is because there isn’t one. Fonts belong in the printing industry and are only defined and registered in a printing context. Aircraft would have been sign written. There may (I repeat may) have been guidance published by the ministry. As it would be directed at RAF ground crew it would have comprised a grid of squares with the letter superimposd. In much the same way as the DfT used to define the lettering for traffic signs – and may still do for all I know.

Since you appear to be only looking for a ‘P’, wouldn’t the pragmatic solution be to acquire a second decal sheet similar to the one you have used? Or is it not that simple?

You can only get to Croydon Airport by bus as it’s a long long walk from a railway station.
I’m afraid I don’t know a bus number so you’ll have to go on the TfL website to find out – use Imperial Way as the destination address.
I can’t help regarding the visitors’ centre – I didn’t even know there is/was one! So much for their publicity given that I live in the borough.