Your issue here will be power loss on the cable too. If it’s nominal 20mm SWA, most likely to be 3 core 10mm or 2 core 16mm using an earth spike at the garage end. You’ll get a ~10v drop with both on that length, ideally if it is 16mm 2 core then you could go to a 40A breaker where the current 30A is as it is still in spec for the size cable.
