I'm also on the look out for free/cheap craft groups in Brighton. I've found a few specific ones (eg. for dressmaking) but I'm a bit broke at the moment and so I'm looking for something more informal and inexpensive. Let me know if you find anything uluru!
I think most of the places where I get materials have been mentioned already but there's also a fairly new bead shop just along from the original one called Kerrie Berrie Beads, which is worth a look.
Another place I'd reccomend visiting a shop called Handmade on St James Street. It doesn't sell materials but there are lots of lovely handmade things designed and created by local makers which are nice to browse. They sometimes have exhibitions or showcases in the basement too.
Oh and the Phoenix Gallery (by The Level) is currently holding an exhibition called Twisted on contemporary craft. They have an event on this weekend called Twist and Make which promises cake, cocktails, workshops and DJs! Might be fun