Only that someone doing this as an amateur would be quite likely to get it wrong, and the possible consequences could either be very bad or just incredibly irritating. Potential is exactly the problem, as I suspect you know. If they end up with one end of the cable tied in to the PME and bury a rod at the other end of it without properly isolating them then it would cause all sorts of problems. This would be easy to do by mistake if you buy parts from a DIY shed and follow the instructions to make up a gland at both ends of the cable. The potential at the substation where the PME ground comes from is not going to be similar to the potential at the garden shed, and will be far less dependant on the weather. The house might not even be PME, without knowing the installation and being able to run the numbers it's not something that can be guessed at (neither is the appropriate size of the cable, it might need to be 25mm for all anyone here knows). Yes ideally the shed (or especially a greenhouse or metal shed) will be TT but if it's not far from the house, it's a timber shed, and the calculations hold up for disconnect times the best thing is to extend the protection. In my case my brick outbuildings are very close to the house and it would easily be possible to have hold of something plugged in to both buildings at once, so the ground potential of both must be equal. Also I have extended the smoke alarm circuit into the buildings which is wired back into the main panel and not to the outbuilding panel. So the zone has to be contiguous.
Also, as said above don't put TV, phone or anything else in the same duct as power. If you really want some run a second duct 500mm away from the first. In my case I have SWA cat5 and SWA RG59 running to my outbuildings, along with various SWA small cables carrying things like the doorbell circuit, heating controls and other SELV junk out there. It can all be done with a bit of work. The trench doesn't have to be 750mm, only 450. 750mm is for drinking water. If you don't fancy all the digging and you know what's under the lawn rent a Ditch Witch. Expensive but amazing tools like ground cutting chainsaws, designed specifically to cut a deep but narrow trench quickly. If you do the digging, the spark won't charge you as much! Not all SWA is actually designed or suitable for direct burial so a duct may prove to be necessary rather than just a good choice. And don't confuse SWA with SWB, braid is not armour.
I'm not an electrician or part P but I was once a data wireman at a large engineering headquarters complex and I know what is involved. It is tricky to get right without experience and it is potentially dangerous which is why almost all outside work is under part P now. I used to spend my days dragging 320 pair SWA cable that weighs 1/2 ton per 100m through ducts and under roads and buildings, working with the sparks to get power to our poles and boxes so I do understand this stuff and I know how to calculate it all and assess the risks. But I don't have the ticket to let me do it, so I don't because I quite like my house insurance to be valid and not to be in jail. Part P is taken very seriously now.
I expect you also know that PME is really quite a cheap con. I've seen and been expected to sign off installations where a customer supplied with an overhead (and now redundant leading and lagging phases) requested a meter position change. The jointers turned up, scratched there hoops and laid a piece of PME to the new meter position. The customer was earth spiked up OK, but had additional hazards of a steel oil storage tank and copper supply and to add insult to injury also moved the water main so his old standby of a bond to the lead supply was also gone as the alterations were done in HDPE.
The two jointers looped the neutral in the customers supply head to the earth bar, checked the supply and put the fuse back in - they sat and scratched their hoops again when the RCD (300mA) wouldn't stay in. They couldn't see that their own neutral was being pulled up relative to ground - could have been phase imbalance or many other things - their answer? Disconnect the customer's copperbond from the earth block and try again - ah OK now......
Except on problem - if ya tried to turn the fire valve off on the oil tank, you got a tingle - the whole house was floating, including the oil tank, sat on strips of DPC membrane.
I can assure you that a skinny 6mm earth to a good bonding rod or plate is better than a missing 25mm one. If the PME installation is done correctly, then the board is responsible for the bond at the incomer and the cutout will have a neutral link to earth in it. It's the only reason PME can be made to work, it's like income tax, everyone has to be in it.
Regarding data and signal cables with shield or armour, you start yet another game again - ideally you only want to be earthing at one end, start earthing at both, and you can end up with some lovely earth loops and induced signals.
I've spent many happy hours on Profibus networks where the shield of the data cable ideally should be grounded as it enters the panel to prevent damage to PLC's etc in the event of an earth fault.
Anyway, back to the OP, if the guy is hell bent on doing it himself, then better he gets heavy warnings to start with. In my experience people will go ahead whether it's law or not.
I've seen what appeared to be a lovely pyro install on a house, all neat and clipped at 200mm - take the lid off the first box and it was Black and Decker lawn mower cable.
