Any question answered...

Can anyone explain what they were actually for?

As far as I know they were the minimum length required by regulation, but I don't know which regulation. It may have been "Construction & Use", but more likely some purchase tax regulation. Purchase tax operated from 1940 to 1973 in the UK to discourage unnecessary consumption, initially as part of the war effort but later (until joining the EEC made it unlawful, being replaced by harmonised VAT regulations) as a protectionism tax applied selectively to goods categories dominated by imports. It might well have ^{[speculation]} classified cycles as "Utility" with mudguards (no tax) or "Luxury" for racing bikes without mudguards which would have been taxed.