Limit for a single transaction is set by the merchant. It’s more often than not unlimited though. There is no cumulative limit as it’s using biometrics.
And I think the one notable retailer that does have an exception is Tesco. I can’t recall if there limit is £45 (same as contactless) or higher - but certainly low enough you could spend over it on a Big Shop
Edit: There’s talk that this may have changed in 2018, not super sure!
Tesco is £250 for Apple Pay now, so not unlimited but much better than before - I have never spent that much in Tesco! Not sure if it was a swift roll out, or if they are still rolling that out.