When one looks at the potentially unsafe and very visible modifications which some owners have, and have without any apparent issues, despite the legislation requiring IVA Tests, I don't think we need to be concerned about a few toggle switches.
For example, we have excessively lowered cars (not just Minis), GRP flip fronts which have no confirmation of structural integrity, stretched tyres, wide spacers, un-padded roll cages, very noisy exhausts, and much more.
If MoT testers and anyone else with a vested interest (e.g. DVLA, DoT, Old Bill) are not interested in these very easy-to-spot illegalities, then there is hardly going to be an issue with a couple of switches.
On the other hand, so long as C & U regulations are not breached and all alterations are declared to the insurer there seems to be no reason why a car cannot be altered. There are specific regulations which must not be breached, but even these are largely ignored (see above) and it is a risk which many are prepared to take. Let's not over-react about all this. Just be sensible and all should be OK.
It occurred to me yesterday that I moved my mobile phone within the glove box in my BMW whilst driving in order to find a packet of mints which were in there with the phone. Touching/moving a mobile phone whilst driving, even if you don't actually use it, it technically an offence, but I'm sure as hell not going to worry about that one bit. And, yes, I do have several toggle switches in my old Land-Rover Discovery for the spot and fog lights and some internal stuff. Never had a problem at MoT time with this.
Sometimes we worry about unimportant things whilst ignoring more critical aspects of our cars. There seems to be no attempt by officialdom to stop these 'genuine 1960's Mk.1 Minis with MPI bodies, engines and interiors' so a couple of toggle switches are not going to be an issue at all. If you want the better toggle switches, just fit them,