I take it you have looked at the ESQC regs to find out about if the overhead cables are up to standard? There are many types of cable used for these types of services, and the regs dont tell too much about them other than safty clearances and minimum heights etc.
With regard to any cable being put underground to feed others, the supply company, have to abide by the fact that, in the event of a fault they must take all reasonable dteps to ensure that day to day disruption is kept to an absolute minimum,also any groundworks undertaken, are tightly controlled, and the site must be returned to its original state.
If you agree to let them install anything on your land, insist on it being placed in plastic ducts, with the joint holes excavated off your property if possible, then if anything happens in the future, they can pull in new cable without entering your land.