There are lots of contracts in London with 500-700 GBP per day rates...Java/Ruby/Node and more. Those are for dev roles, not consulting roles.
Some of those posts are not even real roles - new guys in agencies need to collect a lot of CVs, so they post bogus £600 contract and collect your CVs as "stuffing" for their "portfolio"
£700 are rates for an experienced core Java developer with investment banking or FX experience.
Occasionally, there are much higher rates (1k-2k) for very short urgent roles, but not that common.
My most well paid contract was awarded to me after the job was open for 3 months - Thales e-Security HSM API specialist.