Don't get disheartened.
You have your isolator now. So you are free to turn of the power, and remove your old CU.
Then fit the new one. The only "new" bit will be connecting the tails from the isolator, into the incomer of the CU, and connecting the earth.
Once you have done that, it's a case of testing each circuit one at a time, and then connecting to the new CU. That should be just the same as testing and connecting the circuits you did on your practical, except this time you are not installing them, just testing them.
Remember, you won't have any power to start with during the CU change, so if you need to drill fixing holes etc make sure your cordless drill is charged before you start. A good head torch is equally useful. Also it's obvious, but start the CU change early on in the day so you have plenty of daylight, and personally I test and connect all the lighting circuits first and get those back on so you have light in the house again.
Before you start, I always do a global IR test on the whole existing board. If that fails, then you know you are going to have some problems to sort out. So if you get poor IR readings do some investigating first.
If global IR is good, then you are unlikely to meet any "show stopper" problems. But you might still find faults like an incomplete ring circuit, or bad continuity readings, which will need sorting out.
The other fault you may find is a borrowed neutral on a lighting circuit (usually landing / stairs light) That again will need investigating, but if you are running out of time and daylight, then for a temporary measure put both lighting circuits on the same RCD, and investigate and correct the next day, when you have plenty of daylight.
Other obvious things are mark each circuit (masking tape and a marker pen) as you disconnect them from the old CU, so you are not left scratching your head with a bundle of similar looking wires.
And don't forget to test / upgrade water and gas earth bonding.
Regarding pulling the main fuse, that's been done to death on this forum. Officially you shouldn't, and certainly in a case where your DNO is obliging and comes promptly to fit an isolator there is no need to. But some DNO's and some energy suppliers are not so obliging. This is where you hear talk of the seal fairy visiting first, but I would NOT advise that approach unless you are 100% certain what you are doing You could be in big trouble if something goes wrong. I particularly draw your attention to some old type metal cased supply heads that can be lethal and I would never touch one of those. In any event, if you do "find" an un sealed main fuse or meter, phone the DNO and tell them what you have found and they will come and re seal them.
