Reading around a possible solution may be to employ someone to fit a new CU and multiple stub circuits as the expansion of an existing circuit isn't notifiable, except for the bathroom of course. Thoughts?
That approach has been suggested and discussed here on a good few occasions and, despite what some people think about the 'spirit' of the notification rules, it's hard to see that it would actually break any of those rules, as they are written.
As I'm sure you know, you have identified the only three types of work which remain notifiable (in England). In fact, as far as bathrooms are concerned, it's only work in bathroom
'zones' that is notifiable - so you might well be able to do (as part of your 'extension of circuits') the lights (including switches), if the room is more than 2.25m high - probably leaving only an electric shower and any shaver socket etc. for the electrician..
You would, however, probably have to find a 'sympathetic' electrician, since I suspect that a good few might be (unnecessarily) 'nervous' about your approach and/or would hike their prices to something a lot more than 'stub' circuits would really justify. They would, of course, have to test (and document test results) for each of the 'stub circuits' in just the same was as if they were 'full circuits', so there might be a little more of their time/cost involved than one might at first think.
I presume that you would have to be careful to ensure that the 'stubs' corresponded to the 'right sort of circuits', with the right protective devices. In particular, if you wanted to have any ring final sockets circuits, I think you'd have to make sure that the electrician installed 'mini rings' (even if only one or two socket), since changing a radial to ring circuit might, in some people's minds, constitute 'installing a new circuit' (hence notifiable).
You probably should also consider what sort of 'stub' could be said to be 'a circuit' (which could be 'extended' without notification) without too much argument. I would think it would be wise for every 'stub' to be connected to 'something', like a socket or a light (via switch), since just connecting MCBs/RCBOs in the CU to nearby junction boxes/whatever might be regarded by some as "taking the pi$$" - hence possibly at least some scope for 'arguments'.
Finally, I imagine that you would have to get at least some of your 'extensions' at least partially 'installed' (but not connected to anything), 'ready to be connected, before the electrician did his/her work, or else you'd have a period with no sockets, lights or anything.
I suspect that some people here won't be 'happy' with the approach!!
Kind Regards, John