Hello,
I don't live in London (I am in the north west) so I cannot advise you on which area are more affordable in London. I just know that £800 is not much to live in London, you will probably have a tiny flat for this amount.
Did you have a look on these website ?:
http://www.ashleymilton.com/
http://www.moveflat.com/My/Maida_Vale/
I think it could be easier for you (and more affordable) if you would share a flat with other people. It will give you the time to know which areas you like and which ones you can afford.
Also I think it could be quite hard to find a flat before you are actually in London. They usually ask for references (proof of income , previous adress, etc) et they also usually will ask you for at least a month deposit (+ the first month) and you will have your deposit back when you leave the flat.
Also if you would share with other people you don't have all the bills to pay !
Here is an example of how much the bills costs:
Water: £200 /year
Council
Taxes (depend in which band area you are) : +- £ 700
TV licence : £121/year
Electricity & Gas : +- $40/month
Telephone/Internet Broadband/ cable (TV) : +- £40/month
The food is quite expensive, it cost us around £70/week (2 person) when we are really careful !!
I hope this helps !!
Joanne