In the concertina context, 'English' is not a nationality, but a playing system. If a concertina has thumb-loops and each button plays the same note on both push and pull, it's an English.
They usually have 48 buttons in total, but some types have 56 or even 64.
In restored condition, the value of the most common type, the 6-sided 48-key treble, will start at about £650 for the most basic brass-reeded type. With steel reeds, metal buttons and 5-fold bellows, you'd be looking at £1200 to £1600, depending on make and model. The 12-sided Lachenal Edeophone would be around £2000 to £2500, and at the top of the tree, the 8-sided Wheatstone Aeola would be worth at least £3000.
Instruments with 56 or 64 buttons will always be worth more, as will those with rare finishes - amboyna veneer or tortoiseshell, for example.
Wheatstone Aeola tenor/treble