Reddit reviews Bats: An Illustrated Guide to All Species

Many of these questions I won't be able to help you since they are UK specific. You'll want to seek out Bat Conservation Trust and other local groups. Certainly you can still walk around in the evening with a bat detector like a BatBox Baton or other model and listen for the differences in their songs. I've heard that with practice, some amateurs are able to pick out different species this way. I can only tell roughly who is a hoary bat and who is either a big brown/silverhaired and who is a myotis in my area. There are a ton of bat non-profits on facebook, start following them and you'll eventually start getting suggestions for some of the smaller, more local ones you may not have been aware of.

I can recommend this textbook as a very good read on bat biology. It's fairly accessible if you have some biology knowledge, but keep in mind that research is always changing and we've probably discovered (or otherwise reclassified) at least two dozen species since the last revision. This is also supposed to be excellent, but I haven't grabbed a copy yet.

I'd also recommend keeping in mind that many of the skills useful for bat field work, are transferable from other field work, particularly birds (although opposite schedule). If you can find an opportunity to volunteer with any sort of fieldwork, don't pass it up. As your instructors if they might know of any opportunities. Anything described as requiring "a good sense of humor" has the potential to give you a good story and build your acumen.