Typically, users of footpaths and cycleways are much more likely than road users to have an Ordnance Survey map in their hands – maps that have been metric since about the 1970’s and which have included the National Grid system (based on kilometre squares) since the 1930’s. The signs on the footpaths and cycleways should complement that system, not work in an utterly alien, incompatable system.
It sends a bad message to our children (children of course being quite likely to be using the cycleways where they are available). These same children would have been learning their weights and measures in metric at school – they need to see that the same system is in use outside school so that it becomes intuitive to them and they can use it in their daily lives. We (as parents) should be doing what we can to encourage that – assuming we want our children to get jobs when they grow up of course….
Our country also needs to earn money from tourism and to encourage foreign visitors to come here, enjoy the place and explore it (spending their money here whilst doing so!). Again, these are people who have no idea what a mile or a yard is: why are we making their stay less enjoyable by foisting out-of-date signs on them?
[ OK – so some tourists are from the USA, but they’d be expecting metric signs everywhere else in the entire world, so why not here? ]