If the rule of law is the foundation of state, then how can parliament
claim a power not bound by any legal restrictions?

My guess is lawyers and judges would like to be the foundation of
State, and Parliment would like to think they are not bound by any
legal restrictions. In reality both must respect the other to avoid
serious problems for themselves, the public and the rich and powerful.
By definition the rich and powerful can and do keep everyone else in
line.   Neil