Power to shed from my garage

Not knowing exactly what you have in place.impossible to guess Think you need an electrician to have a look as it’s not a diy project.

Yes it is possible but would need to know what is Being powered currently in your garage and what load you will be adding in your shed to make sure you are not going to overload the 40amp breaker in your house consumer unit also the size off the swa cable will need assessing for current carrying capacity Hope this helps

If you put a 2.5mm armoured cable from your garage to the shed, then you can supply that cable with a 20Amp circuit breaker from your garage supply. It would have to be protected by an RCD, which should be in your house consumer unit and in the garage consumer unit. Another small consumer unit with an RCD main switch in it would be required in the shed also.

This is a difficult question to answer having not seen the job. However, the size of breaker is determined by the size of cable that is installed, which is calculated based on what exactly is in the shed and what it will be pulling. At a guess I would suggest 4mm SWA protected by a 20a breaker, this should be plenty of leaniensy for an average that consisted of a light and a socket or two.