Maybe try the Citizen's Advice Bureau? Or this website seems to have some beneficial information:
http://www.direct.gov.uk/en/HomeAndCommunity/Privaterenting/Tenancies/index.htmAs far as I know, your deposit should have been placed in a "protection scheme" of some sort. I believe this is the case whether it's a private letting through a landlord, or through an agency.
Also, giving 2 weeks notice of your departure is really quite minimal. In my experiences, it's always been at least 1 months' notice. (Unless there are extenuating circumstances, such as the place is uninhabitable...).
Personally, I'd speak to CAB/review the above website/see if the law favors my position, and suggest to the landlord that you will be seeking legal action. I'd definitely I'd demand the deposit back. However, since a normal notice period is 1 month, I would probably end up agreeing to pay up to half of a month's rent (since you gave her 2 1/2 weeks notice already, this would bring you up to a month), unless you can find someone to take over the rental agreement for you.
However, before you move somewhere new, ALWAYS get a written contract. If there are going to be any changes to the contract, get those verbal agreements written down, and signed by both parties, no matter how miniscule they seem! Upon move-in, take pictures of everything--especially previous damage or wear-and-tear, and email them to yourself/the landlord/letting agency as proof, as this will serve as a time stamp, too.
I'll follow up with my OH on Thursday and report back, as he was in a pretty similar situation before I moved here.