Hey not sure what your asking really as it could be a couple of things....
If your deposit is 6 months worth of rent then I doubt they'd give you that back. (they generally don't pay for deposits on private lets)
But if your saying you pay 6 months in a lump sum to the LL and then they pay you back.... (which i think is what your saying?)
Then according to Local Housing Allowance as long as your entitle to the LHA (and continue to be entitle to it for those 6 months you've paid in advance for) they'll pay you directly every month the amount you can receive based on your bedroom rate (which they calculate for you)
They wont pay you back in one big lump sum like how you would of paid the landlord though.
So for example, assuming your entitle to LHa and if your LHA rate is 400 pcm.
And your rent is £500 pcm
And you've paid off 6 months in a lump sum, @ £3000.
they'll still only pay your £400 every month.
But after 6 months you of accumulated £2400 in your direct LHA payments.
There are exceptions to the rule - they may wish to pay your landlord the monthly payments directly and if the property is a council house they usually pay them direct.... Also if you become not eligible for the LHA payments at some point then they wont pay you even though you've paid your LL.
Of course this is just going off the little knowledge I have and I cant be 100% or Guarantee, best thing to do would be to ask? But I'm pretty sure that's how it would work.
GL xxx