The correct way of installing an armoured cable adhering to guidance would be to dig a trench (depth dependent on the land usage) then part fill with soft sand , then loosely lay the cable(to allow for earth movement), then some sort of capping, more refill then a yellow warning tape then finish refilling. However if you are doing it in your own back garden some might say it is not unusual to cut a few corners especially as you have already proved how tough the cable is lol. What i personally would not compromise on is the yellow warning tape below the surface of the soil
and i would sketch the route of the cable and put it in an envelope and tuck it in behind/beside your consumer unit so that in 20 years time it is still known about. Hope that helps
Edit: however whatever you decide bear in mind that it will have to be in agreement with whoever is going to sign the certificates as any outdoor circuitry is notifiable. I suspect that most part P sparks would not have a problem with signing to 15"-!8" depth and the warning tape.