£500 per hour?

If a party to a multi-track claim agrees an opponent’s costs budget “subject to the right to challenge the rates to be applied on an assessment” does that call for a rates determination by the court at a costs management conference or can rates be left to be argued out at assessment, if appropriate?

Strictly speaking, the budget is not agreed and the likelihood is that a costs management conference will be directed. However, a formal assessment of rates is not part of costs budgeting—the court approves (or does not) the phases and rates are part of that—although comments which the procedural judge makes about rates can be recorded and referred to at the detailed assessment.