This would be incredibly difficult unless you went through a private landlord or long term airbnb rental. As you have found, agencies are pretty strict unless you have jobs/large amounts of cash at the ready/UK credit history.
Good news is that Gumtree has the private landlord search option. You might find a landlord happy to sign a 6mo or 1yr with a 6mo break clause if you can pony up rent in advance. But then you will have to be wary of scams, esp if you are abroad.
It may not be ideal, but have you thought about a short term flat share? Could be a great option to get you started and cheaper than hotels.