My house has a TT supply, no earth rod (as far as I can tell, at least, I can't find it nor any cable leading to one), and the consumer unit is sited at low level inside the integrated high level oven unit. It does have each circuit protected by an RCD but you have to remove the bottom 2 drawers before you can get to right the wrong! I did carry out some loop impedance tests on the socket outlets and gave up worrying about it .... values were mighty high and best forgotten.
Slight digression, new 11 month old puppy went behind TV and bit into cable. RCD actually worked .... we now call dog "Zebedee" as he jumps about all over the place ... he very lucky to be alive
Anyway, I had to make a choice. Work on my own house, take out the kitchen, then do a rewire or find somewhere else. The problem with working on a customer's property is that you are supposed to be insured to do the work and you won't be 'insurable' till you've passed the test so you are officially supposed to go to Building Control and PAY them to act on your behalf and to protect the customers interests. To compound problems further, the customer I had in mind did not want BC involved for personal reasons, so it was decision time ..... Though I don't recommend it, the customer agreed to waiver any external involvement by BC and I went ahead and used this job for my assessment .... and as there were 2 parts, minor works being addition of sockets and notifiable work being a new consumer unit in separate dwelling, the whole thing went ok.
One thing I think you have to appreciate is that any work you do before becoming 'competent' makes your insurance null and void. So, even if you do decide to carry out work at a friends or in your own home, you must make sure that nothing untoward will happen! We live in a World that looks for excuses, don't give them any ....
The economic climate being what it is, I don't know how many electrical jobs I will carry out as I am a suitable competent builder, but I can always plan to do my own home next year if nothing else turns up - at least I'm covered