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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.)
Is it 'allowed' to use an earthed one for a double insulated appliance? Should the earth pin be left NC, or tied to neutral?
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?
Of course.
Should the earth pin be left NC,
Yes.
or tied to neutral?
NO!
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.
[earth...] or tied to neutral?
No! NEVER tie earth to neutral in branch circuit wiring or appliances! Never, ever! (obviously they're connected somewhere, but usually outside the bailiwick of a UK consumer, and even in North America it's one specific place).
Tying neutral to earth defeats the system bonding that makes the earth wire safe in the first place. And the problem is, if you jumper them in the plug, that affects the whole system while it's plugged in - and the receptacle switch does not solve this!
So that leaves the other option. Use a grounded plug and leave the ground pin disconnected, if your codes support doing that.
By the way, for an object lesson in the perils of earthing an appliance, consider the American experience: from 1966 to 1996 dryers and ranges were exempted from the earthing requirements and told to "earth" their frames to neutral (think: TN-C-S to the appliance). Just as John Ward says, this energized the chassis when the PEN wire failed.
