Which map to use for cycling

You can use Garmin Express to look at the map coverage.

The UK is in the EU South West map.

The DEM and Geocode maps can't be disabled. These contain elevation data and POI/addresses/etc respectively.

The base map is a very basic world map. It's used for low zoom levels and to show something for places you don't have a detailed map for,

There's no reason to disable any of the maps.

You get no benefit from disabling maps. (Unless, you happen to have two maps that overlap. In that case, only one of the maps should be enabled.)

Unless you have specific needs the built-in garmin cycle map should be good enough for you. With regards to installing 3rd party maps, which can become a hassle but is very useful if you travel abroad, there is a good tutorial here www.dcrainmaker.com/.../how-to-install-free-maps-on-your-garmin-edge.html

It's easy enough to install 3rd party maps (after doing it the first time!). There's less value to using 3rd party maps then there used to be: most of the maps use the same Openstreetmap data.

The dcrainmaker page is kind of obsolete since it talks about using a zombie site that is no longer being updated (sad, because it was a great site).

This site is current and also very good:

https://extract.bbbike.org/

Thank you

Specially if you use Garmin Basecamp for planning then I recommend (and used since my first Garmin Edge) 

https://www.openfietsmap.nl/downloads/europe

All cycle paths are easily recognizable.

https://extract.bbbike.org/  with the same openfietsmap is for Garmin device only (not for Basecamp).

The bbbike maps will work for BaseCamp. I suppose you are talking about installing the maps?

I use a virtual disk (imdisk), which lets you use maps in BaseCamp by just copying the files.

That's correct, but the installation method with the complete europe map is easier to handle.

It's not that hard to set-up the virtual disk and copying is easy. And you don't need an installer (useful if you create you own maps).

And, if you are using the bbbike maps, being able to just copy them is easier than having to get/use a second map.

People generally don't need the map for the whole continent (especially, if they don't live there). Bbbike lets you create maps for an arbitrary region (it's slightly harder to do that on bbbike than it was on the zombie site). I'm not sure if the openfietsmap.nl lets you do that.  Arbitrary regions are very useful because the map file can be much smaller and it's faster to download a fresh map.

Anyway, the OP was talking about using the maps that came with the device on the device (and probably doesn't use BaseCamp).

Yes, it's off topic...

What about the missing contour lines of the bbbike maps? only available seperately.

I use Mapsource to transfer the map for any arbitrary region in Europe to the garmin device (or to the virtual disk).

The standard Garmin maps have contours. They may be in the DEM map, which is a separate map (which would mean they could be used with other maps). You can turn these contours on and off. You can also do that if the contours are in a separate overlay map. The contour data also doesn't really change (which means it doesn't need to be downloaded as part of a map refresh). So, being separate might be a benefit.

For people not in Europe, they generally don't need the whole continent. Downloading just what they need makes it easier/faster to get current maps (people won't generally want to download 13GB before every trip). I've made OSM changes before trips to be able to have those changes in the map I use on the Garmin.

Its fine to be able to download a whole continent but it's useful to be able to download just what you need.

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Since you use BaseCamp, you might find this program useful:

sites.google.com/.../bc2ge