Yes, that is allowed. The spur can of course only feed one single or
double socket, or one item of fixed equipment.

Wouldn't that mean the spur would be fed from a 32A MCB (since the OP is
talking about connecting the spur to the same MCB as a ring final
circuit),

Yes.

and thus be incorrect? I'd have said that a spur wired using
2.5mm cable should have a 16A or 20A breaker.

No. A shortish length of 2.5mm may be short circuit protected by a 32A Type
B MCB, provided that alternative means are found to protect against overload
current. Such overload protection may be downstream in the form of BS1362
plug top fuses, provided that only one accessory (such as a double socket or
fused connection unit) is attached.

Christian.