Last year, I paid 80 Euros (I live in Germany but the UK price was equivalent) for a combined update package which gave me:
New version of the nav software
Latest maps for all of Europe
2 years subscription for map updates
License for TMCpro
That didn't seem so expensive. Part of the problem is that the sort of drivers who think their sat nav is responsible if they choose to drive into a river are unlikely to consider that they may hold any responsibility for map updates, electronic or paper.
If the councils and government really gave a shit about this, they could be providing a free basic TMC service, as in France and Germany. That would allow them to show roads as blocked or restricted until the nav manufacturers catch up.