Extend direct burial electrical cable

There is a direct burial electrical cable running from my house to the shed in the back of my garden. This had been put in years ago, was inspected by a certificate electrician and confirmed as correct/valid installation. If I had my ways, it would have been put in a conduit, but it wasn’t.

The cable is connected to a separate 30A RCD in the CU in the cupboard, from there it goes under the floor, runs for about 6 meters to the rear wall, and then about another 20 meters direct burial under ground to the shed. It then rises straight into the shed from under the shed’s floor and connects to a sub-panel in the shed. In the shed, this powers a light fixture (led flood light – 20W) and a receptacle, which in my 10 years in the house has only been used for electric lawn mower and some hand-held tools (e.g. angle grinder).

The shed has lived its useful life and I will be replacing it soon. When I replace it, I want to put the new shed further away from the house and the existing cable is too short – by about 2 meters or so. What are my options to extend this cable to the new shed location? Ideally, I’d like to keep all of the cable underground, but I’m not sure if a junction box can be buried.

To the best of my knowledge, it’s this cable. I am in the UK.