Extend direct burial electrical cable
In the UK, I think the rules are similar to the US and Canada in that you can't bury a junction box, but you can bury a splice, except in the UK you don't call it a splice you call it a joint.
The hardware used to make the splice / joint must be rated suitable for direct burial.  If you search for &quot;resin joint kits,&quot; you'll find something suitable, such as the 3M™ Scotchcast™ Resin kit Series 92-NBA.
Since repairing or extending underground cables comes up all the time, I'd be surprised if they were not readily available in local stores.