That was because UK wanted to be outside of the EU's financial regulations. For obvious reasons I might add, and why the ERG were so vigorous in support of Brexit. Think of some of their members finacial dealings: Jacob Rees-Mogg, Chris Grayling, etc.The paragraph relating to regulation in the City of London is telling: no concessions from the EU in that regard.
But Mr Cameron ran into unexpectedly firm resistance from France on financial regulation. The French wanted to underline was that Britain would not win any "exceptions to the rules of the EU" - particularly in relation to regulation in the City - hence the addition of the "level playing field" line.
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The ERG funding was secretive and tax-payer funded.
Taxpayers’ money is being used to fund an influential group of hard-line pro-Brexit Conservative MPs who are increasingly operating as a "party-within-a-party".
In July 2019 a tribunal declared that the ERG's research must be made public.
The ERG has also been funded by a secretive group called the Constitutional Research Group.