There are the parliament acts which restrict the length of the delay the HoL can inflict (including to just 1 month for money bills). They also cannot amend money bills.

There is however the Salisbury convention which is that the HoL shouldn't block legislation that was a manifesto commitment of the governing party. That doesn't meant they can't amend it at all, but they can't substantively change it. It's also just a convention, not a rule.