Well, there is a minimum on Motorways. However, installing one on other roads would be problematic as the exemptions would have to be extensive and enforcement virtually impossible. Besides, do you really want to speed up huge unwieldy vehicles? Restricting farm vehicles to certain hours would increase costs which would have an impact on the food price.
Besides, the police already have powers to deal with slow moving vehicles if they are causing a major problem.
The Highway Code, I believe, states that slow moving vehicles should pull over to let traffic past, this happens around here. Although not law, this rule can be used as evidence.
As an aside, how much would it cost to install yet more signs and posts on every road in the country (you could probably leave them off unsigned 30mph
roads if a blanket minimum was in place for this maximum speed limit) ? I would prefer this money to be spent on repair work as extra monies would certainly not be forthcoming from the Treasury.
Would any minimum speed limit apply to cyclists and horse riders as they are already subject to maximum limits?