It’s a bit outdated tbh, as it was a reference to to gold colour of the pound coin (as shown in the article).

To be used as follows:

Bredda, set me a nugget. I’ve only got 50p and I’m feeling that Junior Spesh [1]

[1] https://m.youtube.com/watch?v=Q6pbZLiLt30

I use(d) it a lot. Specifically for pound coins, rather than the value of £1.

e.g. you would ask, "Can you swap us 5 nuggets for a Lady?" If you wanted to go and get something from a vending machine, but only had notes.

Even more so than other terms, I think this is being lost to cashlessness.