Correction on my extension leads the 10 amp ext leads (10 & 20 mtetres long) that I made up have 1.5 mm Sq conductors and the 15 amp ext lead has 2.5 mm Sq conductors (10 metres long)My extension leads are made up from flex that has 2.5 mm Sq conductors which is the same size as the wiring to your GPO's. If you purchase hardware store extension leads you may suffer from voltage drop issues because most of these are either 0.75 or 1.00 mm Sq conductors.
If you would like to make your own leads up with bigger conductors than what is available from the hardware store, go to your local electrical wholesaler where you can buy flex in all sizes by the metre and also the socket and plug ends and make your own. Also you can custom make the length you require.