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No. Because there's nothing in the regs book about it.

TN-C-S supplies rely more heavily on bonding of extraneous conductive parts, in the event of the loss of a supply neutral and a concurrent earth fault, but much is made of this 'exporting the PME earth' malarky by folk who have little understanding of what they speak.

As with all things earthing and bonding, as long as you understand what you are doing you should have no problem with your light.

(Edited for typos)

There is no reason PME cannot be exported as Sinewave states.  You do need to take due care to bond all extraneous services, including those in the building you are feeding (just as you should with any other earthing type).  If you look at the NIC technical notes on submains to buildings, you will see reference to 'BEMT' points - Building Earth Marshalling Points.  A terminal in each building where the main equipotential bonding is connected to the submained earth (which must be sized accordingly).

As for your light or socket in the garden - there is no problem with supplying via PME.

There are a few exceptions to this rule where attention should be drawn.  A socket intended for a caravan should not be connected to a PME earth.  If the PME neutral fails (which they do from time to time, either at the service position due to a loose/overheating terminal or out on the network/up a pole etc), the earth of the installation will rise in potential, potentially upto full phase volts.

Having a metal caravan with the chassis live is not good - especially if you are standing bare foot on the soil with a hand on the door.....

Similarly with these pre-fab metal garages.  It is considered good practice to not use a PME supply to feed anything within these.  A metal clad socket fixed to the metal frame would allow the metal garage to become live during a fault.  The majority of these garages are fairly well insulated from the true earth potential by concrete foundations - the concrete may lower the potential, but will not sink it completely.

He's spent too many hours on the Screwfix Forum Bless him!