You need spur- bearing varieties to train as cordons as you need to be able to keep side shoots trimmed short. The RHS recommend you grow Bramley Seedling as a bush or spindlebush (basicallya pyramid shape). You will need to have two other varieties nearby (dont need to be in your garden could be in neighbouring area) as it needs these to ensure pollination - its a good cropper though so even if pollination isn't 100% you may get enough of a crop for your own use. If you have several maybe you could try one as a cordon anyway, just be prepared that it may not crop very well.