Obvs you can't get very close at Stonehenge, but Avebury allows you to fondle up close. From what I can tell (geology degree from nineties) the sarsen rock is very similar or identical to Fontainebleau gres, i.e. calcified sandstone.
Not been been here or to the Mother's Jam, which is the source traditionally quoted, but formation appears to be similar too in fairly thin beds with uncalcified easily eroded sediment either side. However crucially and unlike Font the max thickness of the beds appears to be less than 2m, more typically 1m, with the max stone size determined by the beds. So for bouldering highly unlikely to provide anything worthwhile in natural state, but a number of choice pillars erected in some sort of group would likely provide the best problems in the south of England.
