"Pay rise" vs "pay raise"

That depends.

In American English, a person receives a raise in salary. In British English it is a rise.

Source: dictionary.com

In each region you can even use raise / rise without pay and there is no question about what's increased:

Ben received a raise.

or:

Ben received a rise.

Hmmm - just based the general usage of the words in other contexts, I would say "pay rise" denotes some across the board increase that everyone got due to contract negotiations or gov't action. A "pay raise" would denote that Bill alone got the increase.

Even so - it's a stretch and I would normally ask for clarification as the meaning is unclear - I'm not sure anyone else would see the difference. It strikes me as the sort of differentiation that a specialist in accounting or management would make aot the normal guy on the street.