Having never heard the terms "trunk road" or "gritter" it took me a little time to figure out what this was. Correct me if I'm wrong but "trunk road" appears to mean something like main road and "gritter" seems to mean plow and/or sanding truck.
"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/cit…
Yes, this is why it is very useful to know, when navigating out of a UK town / city, the next big town / city beyond your actual destination. If you get on the correct main road, you will usually find signs to your actual destination as you get closer to it. Knowing the main road numbers (A35, etc) as opposed to their names is also really useful as is is the numbers that will be used on the signposts.