Coming from a similar professional background, I think I know where you're going with this but here's my £0.02......
Electrics - All you can do really is a visual of the consumer unit and possibly check the earth bonding, generally speaking anything else is buried in or under building fabric. Don't forget you need a Part P ticket for anything more than changing a socket/switch plate
Water - Using your water tank scenario, if you still have a water tank, check overflow for blockages and ballvalve float valve shuts off properly. Check tasp for proper operation and signs of leakage. Unvented cylinders need their safety valves, etc checked annually - you need an unvented ticket for this
Soil/Waste - make sure everything drains properly, there's not much more to it than that
Heating - Annual boiler service and check controls are functioning correctly. If it's gas you need a ticket (Gas Safe), if it's oil you don't need a ticket IIRC but good luck with that if you haven't had any training. ASHP and electric boilers you would probably need the Part P ticket to cover touching the electrical side
Gas - aside from the boiler, you would probably only encounter hobs and cookers and I wouldn't touch 'em with yours let alone mine
External water and Drainage - WALK AWAY NOW, THERE'S NOTHING TO SEE HERE APART FROM TRAGEDY AND COST...leave to the specialists
With the exception of the last item, all the above could be done by a competent plumbing and heating engineer with the necessary electrical ticket to cover controls wiring.
Finally, aiming at the non-DIY but have the necessary cash is why this man is worth circa £70 million.....
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Yeah, I know, it's London and you wouldn't be aiming that high but it was worth a laugh
I spent a few years running a maintenance division with a £1.5 million turnover in the1980's so you don't need me to tell you that 99% of all phone calls are people's problems and the remaining 1% is Kayla ringing to say she loves you and can you pick up some milk on the way home!