hi I read your  post about your Bridgesii cactus , as some one posted  your cactus don't look like a  classic  Bridgesii..  but  that doesn't mean  there not an  interesting teacher

Wow !they are just beautiful cactus .. unusual ,they almost look like a cross of a Bridgesii and a Pachanoi..

Did  you grow them as seeds or  buy cuttings ? 

where they  supposed to be a named cultivator ?

I was going to offer a trade .. but then I saw you were In the UK  , our government in the U.S. is very  fussy about sending plants ether way . Ha

If you try your cactus please let us know if there potent ?

I have no personal  experience  eating cactus or doing extracts .

BUT....I do have experience cultivating ..

I can offer advice on how to harvest a section ..  for consumption so you can continue growing your plants .

MY SUGESTIONS ...

start  with just one of your largest plants .

First  before you harvest do not  water  for  2 to 4 months .

Using a very sharp knife cut 10" or 11" of he tip of the Colum  , dust the cut with sulfur.

Set this tip cutting aside  to dry/callus  over ..  you can root and replant this cutting  later .. store cutting in cool  dark dry place rolled loosely in news paper ends open until it is callused  and your ready to replant the tip.

Then cut a 12" section for your consumption .

when you  make the bottom cut do it at an angle  dust the cut on stump with sulfur .  This stump will  send out pups .  Later you can root the tip cutting .

I suggest you avoid chemistry extractions of just mescaline to start with ,

just use a simple water or alcohol tech  so you will get all of the different alkaloids .

Use all of your 12" cutting ,except remove the waxy film from surface .

Now you will  have two living plants to continue growing  and a 12" center cut piece  to try .

let use know how it goes .