The law is clear on this. Footpaths are footpaths only.
I walk as well as MTB, and when out with my family walking I like the absence of horses and bikes. It keeps things more relaxed.
However, I do agree that the earlier comment about "archaic" and "arbitrary" designations is valid. Many of the footpaths / bridleways and to some extent RUPPs were named thus decades ago, and these designations are no longer fit for purpose or relevant.
The Open MTB organisation is working to challenge some of the distinctions, and get some footpaths upgraded to bridleway, or allow cyclists to use them.
As with all these things, patience and hard work may well pay off, but only in time. Before then. it would be wise not to antagonise landowners and other interest groups such as walkers by using paths which are designated just for those on foot, as much as they may be great for bikes, too.
So, if you care, get involved with open MTB, or write to your MP . If you don't care and are going to ride them anyway, then that seems pretty selfish to both walkers, and other MTBers who will be looked on unfavourably because of your actions.
Hope that is fair and balanced enough. .