It made me smile when I saw the pot front left, with a large cuckoo amongst those dainty cobweb types.

Be aware that that mixed planting on the left contains plants that grow at very different rates - the Opuntia on the left will quickly swamp the small ones at the front. Long term it's worth splitting them up but nice to enjoy in the meantime.

It's not very fair on the pondlife if they get all their water taken away in summer or, worse, poured onto a cactus!

Hmmm, not sure. The normal O. monacantha's as tough as old boots, so in theory this should be too, but can't be 100% certain. Won't be as tender as B. brasiliensis though.

13 looks like Parodia (formerly Notocactus) mammulosa.
No 11 could be the Rebutia it claims to be. It looks like Brian Fearn's handwriting (Abbey Brook) but I can't quite make it out!

Brasiliopuntia brasiliensis (just Opuntia in most books). An Opuntia that needs a little more winter heat than most, as you'd expect for a Brazilian plant.

It's a form of Opuntia monacantha that is both variegated and monstrose.

Natives of mountainous Central and Southern Europe. I don't think there are any UK naitive species.

Surely it's a cristate Cleistocactus winteri, formerly known as Borzicactus aurespinus, Hildewinteria aurespina, plus several other names.

I'd say it was Echinopsis chamaecereus itself, rather than a hybrid.