Map scale for GPS - which should I get

I am looking at the Garmin Oregon 400T. It comes with everything I need for letterboxing and geocaching at home, but I'll need to buy maps for my visit to the UK.

I have found an option that has 20 national park maps at 1:50K for around 80 pounds and another option for just Dartmoor-Exmoor at 1:25K for around 120 pounds. Both options also have the Navteq road maps for all of the UK, so it will be doing double duty as a driving GPS and a letterboxing GPS.

Will the 1:50K be good enough for letterboxing? Or do I need the more detailed maps?

Thanks for any help I can get!

Over here we all tend to work off the 1:25 OS 28 map of Dartmoor for letterboxing. Electronic versions for computer systems and GPS systems do include Exmoor as well.