The current carrying capacity of a cable buried directly is very high, in fact higher than cables clipped directly. The limit is likely to be when the cable it leaves the ground.
I don't have a UK reference, but I could try to dig up a Swedish reference. Or the some of our American friends could tell us what the NEC or CEC has to say.
Edit: I took the first reference that popped up. Swedish AXBJ, 5 conductor, aluminium 1kV cable.
Conductor 65°, air 25°C,
ground 15°C, depth 0,7 metres
Gauge.......50 95 150 240 sq.mm
In ground...160 230 295 385 Amps
In air......140 210 275 370 Amps
Just to give an example.
[This message has been edited by C-H (edited 05-13-2003).]