Of course your right, I've not heard of many electricians being prosecuted for not being in a scheme as you've mentioned.
However, it is a criminal offence, not complying with Building regs (Part P), and punishable with a fine up to £5000. It may also be possible that your clients are complicit in the offence, and could also be liable, 'With all building work, the owner of the property (or land) in question is ultimately responsible for complying with the relevant planning rules and building regulations (regardless of the need to apply for planning permission and/or building regulations approval or not)' (Planning Portal).
So if your not complying with Sect 3.1 Part P (England & Wales), you're a very naughty boy, and putting your clients at risk of prosecution. You may also be invalidating your insurance.
So however guiling it is, and it is, its the law.
I suppose the maximum fine for speeding is £2500, but I don't know anyone who's ever had to pay that. There's always that little element of risk but it makes life more exciting Looking at that particular wording it seems that the spark may not be liable at all, with all the onus on the client to comply?