I don't currently have a map pack or anything that you could download to have them all in one download. The server actually has way more maps installed than are available to vote so I can't just copy them all over to an archive either because it would be massive with lots of maps you definitely don't want 🙂
For a start, you might consider looking at the utstats map link at the bottom of the utstats main page and then clicking on the matches column to sort the maps by the most popular:
Then you can use that list to find most of the maps on these sites:
If you come across a map you can't find just post about it and I'll find it for you.
Once you download a map, you just have to put the files in their appropriate places to be able to play them offline: .unr in Maps folder, .uax in Sounds, .umx in Music, .utx in Textures, .u in System.
After installing a bunch of maps offline, you may one day come across a server running a different version of one of those files and get a mismatch error when trying to join it. If that ever happens you can just delete your locally installed version of the file or move it somewhere else temporarily if you want to be able to join the server.
To get you started, here are links to the 5 most popular maps on the London Combo server: