Power to Garden Shed

[flameport]

The plug won't be fixed to anything, but the SWA, junction box and socket at the other end will be.

Using an extension lead for the entire length would not be fixed, but using an extension lead as a permanent installation won't comply with P1.
(neither would SWA, a junction box and a socket left laying around and not fixed).

 

[chapeau]

P1 (to whose who might not know that's the bit of law which says reasonable provision has to be taken to ensure your new electrics are safe) only applies to stuff that is fixed. If it isn't fixed it doesn't fall under the building regulations, so doesn't fall under Part P. Temporary/Permanent is not relevant.