Garage to shed power

Yes this is possible if you have spare ways it the garage cu. A armoured cable will need to be run between the two cu points the size will depend on the length of run and the potential load. The garage board it is supplied from will also need to be RCD protected or have an RCBO fitted.

You havent got an18 amp breaker in your cu.you cant put sockets on a 10 amp breaker if you want them to comply.get a spark to come round and get free quote ,youre well thinking of doing it yourself.

Yes you can take a supply from a spare way on the cu down via an armored cable. The size of the cable and load it can take will need to be worked out depending on the distance from the supply CU. If the distance is substantial you may need to consider a local earth rod at the shed with no connection made to the earthing terminal of the supply board, except for the armor of the supply cable connected to the earth at the supply end. Both the supply cu and new CU should be protected by an RCD or RCBOS.

Powering a shed from a garage increases the total length of the cable to the consumer unit therefore resistance in the cable. Any part P electrician will tell you to be compliant the Zs (total resistance to earth of the cable) needs to be below a certain value. If your value to the garage is low no problem but if not it could push you over the limit and render what you are planning to do as non compliant. In which case a separate circuit would be required. All very doable.