"dennis@home" gurgled happily, sounding
much like they were saying:

There is a minimum speed that vehicles must be able to do before they
can use the motorway.

Which is...?

(With authoritative reference, of course)

Laws HA 1980 sects 16, 17 & sch 4,
MT(E&W)R regs 3(d), 4 & 11,
MT(E&W)(A)R, R(S)A sects 7, 8 & sch 3, RTRA sects 17(2) & (3),
MT(S)R reg 10

Oh, look, a handy c'n'p of the references from the bottom of Highway Code
rule 253. Which, by the way, doesn't mention any minimum achievable speed.

Any links to any of those? The only ones of those I've found online (HA80
s16, s17) also don't contain any mention of a speed.

I notice you conveniently forget to mention what that minimum speed is.

So - what is this minimum speed?