County or high court claim? England.

Liability has been agreed however quantum (over £100k) is in dispute. Do we start in county court and let it be moved up, if so how is the initial claim form filled in (quantum unknown?).

Although liability is agreed they may still dispute (after all why not, it worth it for them to try and defend). Is there better understanding of DPA, GDPR etc in high court, is it just wiser to lodge in high court?

Finally, liability of other parties costs if being dropped down a court, is that an issue?