Apple Pay limit?

Hi everyone, I am new to Apple Pay as I have just bought an Apple iPhone and Watch. My friend seems to be able to take their Watch everywhere and leave their physical payment card at home.

I have two questions:

1) Is there not a limit like contactless here in the UK, or can you make a purchase of any amount using Apple Pay in store?

2) Every now and again when you use contactless here in the UK, sometimes the transaction is declined due to extra authorisation and security checks being made. Does that happen with Apple Pay often?

Cheers!