Although the garage consumer unit can handle 40 amps, this is more than I
need (and the sum of the individual MCBs is a lot less than this anyway)
so I intend to use a 32 amp Type-B MCB at the house end of the
connection - which seems more appropriate than a higher value if relying
on the 2.5mm^2
earth within the existing T&E cable.

Don't be so sure. You want the house end to blow as slowly as possible to
get maximum possible discrimination with the garage CU. I would consider a
Type C MCB, or even an HRC fuse. If 40A is allowed as well, then use that,
too. I haven't done the calculations so can't tell you what you will get
away with. This way, any fault is more likely to be picked up at the local
MCB in the garage.

* Use 3-core SWA cable, and use one core (6 mm^2) as the earth - and join
it onto the end of the T&E's 2.5mm^2 protective earth conductor - thus
exporting the earth from the domestic CU

I'd just do this given you are only 1.1m from the house. This is only
possible if the garage has no main equipotential bonding requirements, or
the main equipotential bonding can actually be achieved.

Remember to use the armour as earth as well, both to get the ELI even lower
and because the armour must be earthed anyway. The reason to have the
additional copper conductor is that copper doesn't rust at the termination
and is more reliable over the decades.

The most convenient solution would be to have the joint inside a deep
(metal) socket box high on the wall of the en-suite
bathroom - sunk into the blockwork, and covered by a flush blanking plate.
The T&E would come down from above, and be chased into the plaster and the
SWA would go straight out through the wall from the back of the box - with
its gland fitted to a knock-out in the box. Does anyone see any problems
with this?

No problem.

How should I physically join the cables? Is a large chocolate block
connector (inside the box) ok - or is there anything purpose-made which
would be better?

Chocolate block or crimp is fine.

Do I need to earth the SWA's armour even if it isn't being used as the
protective earth?

Yes. Either earthed by using a metal box, or using a decent earth ring, not
the ****e one that you get with the gland.

Christian.