Jeans and a tee are standard for almost any gigs* and remember, avoid a coat, because you will have to hold it, and it could get lost in the crush. Try and wear a top layer you can tie round your waist - jumper or hoodie are perfect. Don;t take a brolly - they won;t let you in with it, and you have to queue at the end to get it back - royal pain.
* Reggae gigs - if by serious reggae artists - are a serious night out, the largely black audience will be dressed to the nines, and apart from being white, you will stand out massively if you have your casuals on. Just a thought - it's a free country, but if you want to make the effort, everyone else will as well.