Sink and toilet combo tap

Most likely his Right if his qualified plumber

Replacing a tap on a sink in a toilet-sink combo unit can sometimes be tricky, especially if the plumbing design is compact or there are accessibility issues. Why the Plumber might be saying that the toilet needs to be disassembled, it could be due to a few possible issues: Space Constraints: In some toilet-sink combos, the plumbing may be very tight or difficult to access, and parts may be obstructing the tap replacement. Leaks or Damage: The plumber might be concerned that disassembling part of the unit will help prevent future leaks or damage, especially if other parts are interconnected. I hope this helps and without more facts of the route of the problem. Having done these jobs it is the last resort but sometimes it is the only way to fix the problem correctly.

He is correct, they can be a nightmare to work on as the supplies are bottom fed and impossible to to disconnect whilst toilet is connected