I just made a call to the family.

I very much doubt there were £100,000 plaques. The highest denom chip was only £500. The destruction paperwork in Victor's estate showed the qty's made and destroyed. Save for a couple of £10's everything was accounted for.
There was a £25,000 but no £100,000 or £10,000.

Btw, there were £5 plaques. That was confirmed by the Grosvenor training school who used them for a while when they opened. They were already cancelled when they received them but I don't know how. Every one was supposedly destroyed.