There are a number of reasons why wages might not rise that have nothing to do with free market supply and demand. The most obvious is you could control wages by being the owner of inelastic goods your workers require to live, such as housing. I think this is how the feudal system worked originally, workers produced cereals but the Land and the worker's cottages were both owned by the Landlord. If the price of bread…
Student loans are paid back based on salary. 400-300 a day is low for a senior software contractor in London, or even a FTE (hard to judge difference given benefits).
6-8k a month is a good wage on London.