If you just need the power for the ONT 1/2 a metre longer then you should be able to use an extension on the 12V side i.e. between the existing power supply lead and the ONT. You can buy 2.1x5.5mm extension leads in various lengths (often used for CCTV) from the likes of Amazon, Kenable, Ebay etc. Much smaller and neater than extending usibg a 13A socket (cheaper too!)
Make sure the cable you order is the right size for your ONT - different models may use different sizea.