Here is the keeper
standing-up to a young Jimmy Anderson (from the 3 minute mark) in a T20 game against Australia. Anderson doesn't look like he's holding back, so I assume that he's bowling in the mid 130s. That is, fast-medium.
Having read about, and seen the footage of Gregory, Tate, Allen, Farnes, Voce, McCormick, and Wall, I don't believe that there is much doubt that they all operated consistently in the mid 130s, or higher in the case of Gregory.
Bill Bowes is interesting as his action makes it difficult to judge. He did concentrate on swing, and I am reminded of Damien Fleming, so maybe around 130?
Larwood would have pushed the 140s, and on a good day hit the 150s.