I think i made myself perfectly clear what the commonsense approach would be for a dwellings garden!! It's certainly not 250mm.... A minimum of 2 X a garden spade or fork length, or 600mm!!
It's also important to point out, that whatever the trench depth is, it should be free of any sharp stones and other debris. And should have a good layer of compacted bedding sand to lay the cable onto, along with a decent covering of sand before a relatively stone free soil back fill. It's highly unlikely that protection tiles are going to be utilised in a garden install, but a warning tape should always be provided at around 150mm below ground. Personally i always carry out a sheath test on all direct buried SWA cables before terminations are made...
I do however agree with you on some of the other methods being suggested here!! ...lol!!