Not my nonsense!Why pay for an RCD when plugging it in to an RCBO protected socket. Where do you get your 12 metres from that's nonsense
Table 15.4 on page 98 of the 4th edition of the IET Code of Practice for in service inspection and testing of electrical equipment quotes max lengths for extension leads.
In any non-domestic context, the issue is that users without your technical knowledge could use the lead without the required additional protection, putting themselves and others at risk under fault conditions.