1. Can he utilise a spare 32A MCB running through a 80mA RCD on my CU
in the garage?

Yes.

3. What size of cable would be required on this 55m run with, what I
assume will be, a huge voltage drop?

Indeed. And you should really allow voltage drop between the garage and the
main house CU as well (unless the garage CU is located at the service
point). Also, there is voltage drop within the shed installation.

Assuming that there is no additional voltage drop to take into consideration
and allowing 10m within the shed, 10mm SWA is indicated for the full 32A
load.

If you design for a lower load, such as 20A, which will fulfill all your
needs, you're on the margins of being able to use 6mm.

4. Can he install a switch within the kitchen to turn off the supply
when I'm not in the shed? (This is to avoid the local thieving chavs
nicking my juice!)

Yes. Use a 45A DP switch for this. Often sold as cooker or shower switches.

5. Can he install T&E through the house and changing at a weatherproof
junction box on the external kitchen wall, to save on the cost of using
Armoured all the way?

Yes.

6. Will he insist on a separate CU in the garage running from the
meter? (More money though to get electricity board involved)

No.

7. Is it necessary to have a CU with separate breakers in the shed to
control/isolate all the different components?

No, as it runs from an RCD circuit anyway, but you might as well. There's a
Volex "garage" unit, which has an RCD and a couple of MCBs in a waterproof
box. If you don't have one, the lighting circuit must come off an FCU, as
32A would be too much without further protection.

Any advice would be welcome to avoid being taken for a ride.

It may be inappropriate to export the house earth this distance. You may
need a TT earthing system (earth rod) and RCD, with the house earth (i.e.
SWA armour) isolated at the shed consumer unit.

Christian.