As others have said, it's completely standard practice to give a new tenant only a 6 month tenancy. The LL doesn't know you, and therefore they don't know if you're going to be a difficult tenant.
It's in your interests too, especially if you've paid the 6 months in advance. (Which is again standard, I'm afraid, in instances like this). If they don't do repairs promptly, or if they breach access restrictions, you'll know they can't be trusted and you need to find a new LL who can. Also, if your circumstances change dramatically and you can afford a bigger place again, you're not tied in to this place. If the rent does increase, you've got the opportunity to move to a cheaper place (though I realise this is not much of a consolation if you're looking for stability).
After 16 weeks or so (if your LL is a good one, and organised), they'll discuss renewing the AST on its current terms, or agree to just carry on as before - so the tenancy then becomes a periodic tenancy, and you're not restricted to a fixed term. From that point onwards, your LL must give you two calendar months' notice served on a rent day; you need give only one month's notice. Again, probably in your interests if your circumstances are liable to change.
One thing - if you're planning on paying for subsequent tenancies with housing benefit, you must make sure that the benefit cap amount at your local tenancy will be enough to fund your entire rent. If you get into arrears, (even if you later repay them) it becomes almost impossible to rent in the private sector again. Apply for HB straightaway - the process takes a while, and money can't be backdated.
Good luck with it all and get straight back on here if you experience problems as a newbie tenant - there are loads of experienced LLs on this forum who are willing to help.