One of the issues (in the UK) is that, although we could, given adequate information, advise the OP about how electricity should be supplied to a domestic outbuilding (primarily in relation to the nature/routing of the supply cable, and maybe the 'earthing arrangements'), there is no control over what householders can (and do) do with multi-outlet plug-in 'extension leads' (which are very widely available and used).I am somewhat amazed that a "set-up" such as this as proposed would be allowed anywhere in the UK. (It would not be allowed in several other countries of which I can think !)
Hence, if someone plugged such an 'extension lead' into a socket somewhere in the house, and used its outlets in an outbuilding, there is, as far as i am aware, no way that could be 'not allowed' (even if 'not advisable') in the UK.
Kind Regards, John