There is no requirement for the fused isolator to protect the DNO side, that have their own fuse. It is so you can justify protection of the sub-main without relying on the DNO's permission.
True it won't provide selectivity at the DNO if you use the same value (assuming it meets the sub-main disconnection times) but if you are likely to blow the DNO fuse without a major sub-main fault (pick-axe through it, etc) you have under-specified the supply in the first place!
That is why I suggested a 60A fused-switch at each take-off point, as that should provide selectivity with the incoming fuses of both DNO and a customer.