"Main road" is actually considerably more general than trunk road in the Great Britain. The roads in GB are ordered hierarchically starting with motorways (although these have restrictions, ie. motorvehicles only), through trunk roads, primary, secondary, all the way down to byway.

It is important for navigation in GB. When you want to go somewhere you are supposed to use trunk roads to navigate to the first town/city on the trunk network beyond where you are going. The signs are coloured to indicate you are on the trunk network. Then as you approach your actual destination you will begin to see it signposted on the local non-trunk roads, at which point you leave the trunk roads.

This has been mostly forgotten in this age of GPS navigation, though, and many signs are in a poor state of repair or badly occluded by trees or shrubbery.