Hi Simon
Nuts are not normaly grades in the same way as bolts, for general purpose engineering anyways.
Most nuts are usualy grade 8 .... These will be fine for use as a captive nut for the seatbelt mounts.
As far as I am aware, there is no IVA requirement to have 8.8 stamped on the nut, only on the bolts.
Most bolt stockists should keep those nuts in stock. ( Fairly common imperial size )
If you have trouble finding them, my local steel holder keeps them
