I find TomTom more user-friendly than Garmin - it's all a matter of personal preference, so the only way is to go somewhere like Halford's and play with them.
I've never seen one that allows you to specify OS grid references but Lat/long is common and most have "Places of Interest" eg, tourist attractions, car parks etc.
The more expensive they get the bigger area of mapping gets, so start by looking at the cheapest since you only want UK maps. Lifetime map updates is worth considering, as buying them as you think you need them can be expensive.
Go to Halford's and try programming one to plan a route somewhere and then try to make it go via somewhere else to see how easy that is. eg, we live in the south of England and regularly travel to the north and most sat-navs want us to use the motorway to the east of Birmingham; we prefer to go west of Birmingham so set a waypoint on the road to make it go that way. Repeat with a different make and see which you prefer.