I currently have power to my garage which is fed from a 30amp fuse on the
main consumer unit to another fuse box in the garage. This fuse box has 2
feds one for a small socket ring (3 double sockets) and the other to provide
lights for inside the garage and security lights. One of the double sockets
has a built-in circuit breaker and is used to supply power to a small pond
pump and light (this is the only constant draw).

In addition to the garage I plan to build a big shed to be used as a small
scale workshop. Rather than take a new feed off the main consumer unit I was
planning on extending the ring and lighting circuits from the garage to the
workshop. I will be using a few power tools but it is unlikely that the will
be all used at the same time. Possibly use a dust extractor & saw or router
at the same time. Can not see that there would be any greater a load than
would be found in a modern kitchen.

Do anyone think there is anything fundamentally wrong with what I planning ?

Eric.