Hi Beowulf03809, they are custom and are simple to do. The theory is that anything that might be used for skirmish/other games with different basing gets based the GW movement way now, while everything else (which is most of my LOTR figs) gets permanently fixed to bases. This is mainly the Vikings/Dunlendings but I am also doing my Celts/Wildmen this way.
So those bases are just wood cut to size, with gapfiller spread over them (Selly's no more gaps or Bostik gap-filler are the ones I use), the gapfiller is a is rubbery when set so is very durable. Then I stick the figs in that, along with the other basing bits like the rocks, branches, flagstones. Paint with sand + paint obviously, then statc grass, flock. I am not very adventurous in that regard, but if you have a look at some of the basing on Craig's Wargaming blog and Kruger's Creations, you will see some much better tips and nice examples. I am the 'rough' wargamer out of our group.
We do like how it lets us vary the formations. TO handle Epics, we just design clearly marked 'command companies' with space for the Epic to be placed.
I also like how it protects the figs. A large base is much more stable and you can ensure that most (hard to do with some cav) fragile bits do not overlap the base edge. We mark casualties with a set of smaller dice. Setup is easier too. I find using the process above to be much quicker than doing individual 25mm bases for the figs, then doing up the WOTR tray. I just get a group of bases, stick on the layer of gapfiller and just press everything into that. I am getting the hang of making the bases look better, the Gondor ones were my first try. I like the Rohan Cav unit I did better.