The frame is 23.5 inches in old money. The modern trend is to have a yard or so of seatpost showing, in the old days however this was not the case and seatposts were much shorter back then because of this. I am certainly no daddy long legs - one of my bikes is a 23.5 . I am just under 6 feet in height (with a 34 " inseam). The most important aspect of frame sizing is reach - a small frame will be correspondingly short in the top tube, a long stem to compensate (over 12cm) does not a good fit make.
British Eagle (of Newtown, Powys - part owned by Barry Hoban) did have a bit of rep for <building frames slightly out of track>, this bike comes with pretty mediocre components and a frame which certainly is NOT superior to 531 IMO