I'm an English Morris dancer and play mainly English tunes in the harmonic style (tune underpinned by chords, basses and harmonies) which means most of what I play on a CG box would be in C or G. Irish players tend to play single line melodies with only minimal accompaniment. This allows them to roam free across more keys, often playing in D or A on a C G box. To do this, they need more than 20 buttons, and 30 is the norm.
Mikefule, I hope I am not hijacking this thread, but do you feel a 20 button anglo is sufficient for most English tunes? I don't play the concertina yet (I am learning the D/G melodeon) but I would eventually like to acquire a C/G anglo. Would 20 buttons be sufficient for playing harmonic style arrangements of English tunes, possibly including accompanying songs? My top musical "role model" at the moment is John Kirkpatrick, and I believe he plays a 40 button anglo and makes use of them all! But of course I'm not looking to play professionally or revolutionize the folk world, just to play for my own enjoyment.
Certainly, there is a lot that you can do harmonically on a 20 button, especially if you take the additional trouble to play across the rows in the higher of the two main keys (G on a C/G box).
Of course, the 30 button offers more and easier accompaniment options. I also have a 38 (unfortunately B flat/F so useless for Morris sessions) but I only tend to use the extra buttons for non-Morris tunes.
I believe it to be the case that William Kimber learned on a 20b and when he was later given a box with more buttons, he stuck to using the core 20 for his Morris tunes, although he was perfectly able to use the others for his wider repertoire.
If you want to play with other Morris musicians you will find that G is the predominant key due to the melodeon.
Playing in G on a C/G Anglo can get a bit squeaky on some tunes. However it is great on tunes that go down below the tonic.
Playing in G on G/D box has more limited chords - I especially miss the E minor push.
If you play solo and put the work in, it's very rewarding.
Most Morris tunes are in one straight key with no accidentals. The same goes for generic country dance tunes. However, many Playford style tunes need the extra notes of a 30 button.
To me, the big thing about a 20 b is it makes you work with the core Anglo layout and it emphasises the quirky character of the Anglo sound: sometimes rich harmonies, but often sparse and sometimes limited to raw octaves, fifths or thirds. It makes me understand the music better. However, there is no doubt that the 30 (or more) button is more versatile.
Here are a couple of my videos of English tunes on 20b. Other and better musicians are available!