Whilst English session tunes may only require the 3-chord trick, the idiosyncrasies of the melodeon, which tends to be the dominant instrument, means that they don't necessarily come in the right order. When I have come to arrange tunes for a band, I've often found that the chord sequences the guitarist wants to use are quite different from those the melodeon demands.

It's not just the chord structure, the somewhat loose rhythmic nature of English music is also difficult for some guitarists. I think it takes an particularly good guitarist to contribute to an English session - I know some extremely good players who just don't "get" the rhythm and so I find it almost impossible to play with them.