Anything up to the property boundary has to be done by the network operator or their sub-contractors. Inside the boundary you (or anyone else you choose) can do the work such as digging the trenches, laying the pipes and cables etc. but it must be done to the network operator's specs. Their requirements for depths etc. will be on their websites and once you've got the pipe or cable in then they'll send an inspector round. The exception (for obvious reasons) is gas piping, then they need to do everything up to and including the meter, but you can still do the trenching inside the boundary.
It can get ridiculously expensive, but they will all give you a quote before they start work. Sometimes it works out as cheap, and much easier, to get them to do the lot. Unfortunately it's a bit of a lottery as they all have their own rules, requirements, charges and paperwork systems. :RpS_unsure: