If you just want to try it out then don't spend anything yet and arrange some friendlies with a borrowed team.
Once you decide to invest the only thing you really need to play at the club is a team, as there are boards, dice etc. in the cupboard. I imagine that you'd want you own copies sooner or later though and then the GW box set has you pretty much covered. There's an old post on BoardGameGeek here where someone lists out the things you need to play Blood Bowl and where you can get them other than GW (it's quite a militant anti-GW post). I'm not sure if it would all come in at under £55 though.
I would point out though that the Orks in the GW box set don't have the right number of some miniatures to make the most competiitve team so you'd be at a disadvantage until you could buy at least two more Blitzers and two more Black Ork Blockers.