You don't have all your rights until you've been employed for 12 months so it doesn't matter whether they call you probationary for 6 months of it. They could still sack you in month 7 or 8 (unless they're sacking your for racist, sexist, etc reasons) for any reason they want.
The 6 month clause is quite standard. If you've got confidence in yourself and you fancy the job go for it. I took up my present post unaware that the 6 month probationary period was in the contract. I was a bit concerned at the time but I'm still here - 11 years on.
Good luck whatever you decide.
