On a serious note, what if one were to enclose a BS951 clamp in an epoxy-filled box - would it still need to be accessible?
I probably wouldn't bother anyway, but I do have this accessibility problem. The plumber who installed the towel rail in the bathroom used plastic pipe under the floor. Doesn't bother me - I'm not a copper purist, but the runs are quite short (30-40cm), so I really ought to bond to the metal pipes that then emerge from the floor.
But the flooring to go down, whilst not tiles or glued, will be hidden-nailed boards, and won't come back up without damage, so I wouldn't countenance that in a hurry, nor would I try to argue that the clamps were accessible.