as often, in thinkdefence.co.uk:
https://www.thinkdefence.co.uk/2011/...ging-supports/
So these are "Christchurch Crib bridging supports" or simply "Christchurch Cribs"
More on this page (search for 'Christchurch'):
https://www.thinkdefence.co.uk/2011/...tion-bridging/
where we (or at least, I
) learn that its offical name is "Bridging Crib 20 ton"
Michel