Although I don’t do extra work myself,I have quite a few friends who are involved in that area of the biz, and from what they’ve told me in conversation, it seems that it’s a good idea to register with as many agencies as possible if you want to keep in regular work.
In regards to paying an agency to appear on their books, I wouldn’t normally reccomend going with them. However, if you need to get some photos done anyway, £100 is roughly the going rate for a set so it would seem to be ok. You should however make absolutely sure that you can use those photos for other publications such as “Spotlight”, as you will require the permission of the photographer to use the photos in such publications.
Good luck with your career!