Is there a maximum length for a spur cable?

A fused spur (or even an RCD spur, boo hiss)  from an existing ring could serve a couple of sockets in a small shed, and maybe a light switch as well, without breaking any rules (or the bank).

For the sort of shed that you can only get one person into at a time, to  use a complete way in the CU is a bit much. however, a means of indoor isolation (the fused spur in the house) becomes very important if the shed is likely to leak or if there is a risk of electricity theft when going on holiday. Similar thoughts apply to external sockets, I imagine you'd not include the area of the back garden in the 100m square calculation, just because a lawn mower may be plugged in once a fortnight in summer. One may think about  relaxing the voltage drop thing a bit as well - if the light in the potting shed dims when the lawnmower  is on, I cannot see many folk caring.

Even if the supply were a 4mm 32A radial I might put a fused spur with a switch as a light switch, rather than a mini CU for separate lights and power if there is only one light switch needed anyway.

No need to introduce another load of connections to test and maintain.

A fused spur (or even an RCD spur, boo hiss)  from an existing ring could serve a couple of sockets in a small shed, and maybe a light switch as well, without breaking any rules (or the bank).

For the sort of shed that you can only get one person into at a time, to  use a complete way in the CU is a bit much. however, a means of indoor isolation (the fused spur in the house) becomes very important if the shed is likely to leak or if there is a risk of electricity theft when going on holiday. Similar thoughts apply to external sockets, I imagine you'd not include the area of the back garden in the 100m square calculation, just because a lawn mower may be plugged in once a fortnight in summer. One may think about  relaxing the voltage drop thing a bit as well - if the light in the potting shed dims when the lawnmower  is on, I cannot see many folk caring.

Even if the supply were a 4mm 32A radial I might put a fused spur with a switch as a light switch, rather than a mini CU for separate lights and power if there is only one light switch needed anyway.

No need to introduce another load of connections to test and maintain.