Firstly - try your doctors - if they do Slimming World on Referral in your area, and you have a BMI of 30 or more, you may be able to get free membership and up to 24 weeks of group.
If that isnt an option -firstly - membership is pretty much ALWAYS on offer, and you dont usually need to waste money on a mag to get the offers - its just advertising.
Your first week will usually cost you about £10 - thats either £10 membership + first week free, or it will be half price membership £5 plus you pay your first week (£9.95 in total)
At the moment (during March) there is a free membership offer on when you buy a 12 week countdown - buying a 12 week countdown gets you 2 free weeks as well (12 weeks for price of 10). But if you are that skint than paying up front probably isnt an option for you (and you are only really saving £5 on membership - not £10 as they suggest, as its always half price anyway). If you do go for a countdown though, remember to book holidays first or you will lose them. A 6 week countdown also gets you a free week (6 for price of 5) but that doesnt include free membership.
I seriously dont know how they get away with calling some of these things "offers" because they are ALWAYS on offer! I've been volunteering and taking the money since May last year and have NEVER seen it full price in all that time!