I haven't been able find a rewirable unearthed (as in 3-pin, one of which plastic) UK mains plug. (I didn't think before hand, but I suppose they're not allowed by BS1363 since you could then too easily fit one where earth should be connected.)
That is correct, "insulated shutter opening devices" as the standard calls them are only allowed for non-rewirable plugs. Presumably for the reason you suggest.
Is it 'allowed' to use an earthed one for a double insulated appliance?
Should the earth pin be left NC,
or tied to neutral?
Neutral and earth should never be interconnected in ordinary portable equipment, or ordinary UK domestic electrical installations for that matter. Neutral and Earth are normally connected somewhere, but in the UK it happens on the electricity supplier's side of the system (this differs from the USA where it happens in the customer's main panel) not within the customer's installation.
The only exception you might possibly run into as a DIYer is when you have a supply that is separately derived from a transformer, generator or inverter. In such cases it may be appropriate to tie neutral and earth of said supply together in exactly one place.
There are other exceptions in certain specialist non-domestic situations, but they are outside the scope of what a DIYer could reasonably be expected to encounter.