"It is theoretically possible to play songs in other keys with the standard configuration concertinas but it does require learning new fingerwork patterns, some of which are awkward."

Funny how often I see or hear that said about diatonic/chromatic instruments.
It's like saying a piano keyboard is best for playing in C major, and it starts getting tricky in other keys.
Exactly the same is true of almost every instrument ever invented. The only difference is that to play classical music you have to learn all of those finger patterns, while in folk music you can get away with not bothering.

I'm quite lazy with the Anglo. Sometimes I think I ought to sit down with it and practice playing major and minor scales in all keys, going up a semitone at a time, like I used to (and still do sometimes) with the cello. I know I'd get quite good at it if I did it enough, that it would take months to perfect, and that I'd be able to play some amazing stuff eventually.

The only thing that the Anglo can really do better in C, F and G is use low notes as the basis for chords - there are big gaps in the scale at the low end. In the middle range, playing in remote keys is a simple matter of practice!

Similar with my Saltarelle 2½ row melodeon - fully chromatic over three octaves on the RHS, but let down on the chords and basses.
If I could be bothered to learn the fingerings for playing in other keys... some people have.

Anahata