The new Supreme Court is still composed of judges that must be approved by the Secretary of State for Justice (i.e. a government minister, a politician). Technically the appointment is made by the Queen, but overruling an elected minister would be considered an infringement of constitutional prerogatives of Parliament. That means a government can veto high-justices it doesn’t like.
Downstream of that, the judiciary cannot invalidate or overrule primary legislation. Parliament comes first.
What we have in UK is fundamentally a degree of protection of most judiciary elements from the otherwise-supreme power of Parliament. That’s not absolute separation, but rather a partial one - if very extended.
No court in the UK has ever overruled the parliament.