Not an ideal situation, having to bury a join, but it happens all the time, sometimes you can over stress it.
Since you can make that hole as big as you need, I would of used one of these 32 A Ashley MF junction boxes.
Box secures to the wall and good clamps on cable. Used these under floors.
Safe zones is one thing. I would be more concerned about verifying the circuit is healthy to start off with.
That the conductors are continuous, that its not already a spur. Zs, R2 and RCD times are acceptable, before connecting the spur.
But you need a meter !
Then retesting the same at the new socket. Making sure polarity is right. and values were as before, so you know your join is acceptable.
Only then would I be happy to see it buried. Half hour job for an electrician. Save you the worry
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