In reply to BnB:
Convincing as your figures might appear at first sight, I suspected it might be rather more to it than that, so I looked into it and found a detailed report into precisely what we're talking about, from Oliver Wyman, who I presume know rather more about this than either you or I.
http://www.oliverwyman.com/content/dam/oliver-wyman/global/en/2016/oct/OW%2...
This, from the Executive Summary, should tell you as much as you need to know about the expected impact of Brexit on financial services in the UK:
"The UK-based financial services sector, together with the related professional services sector, has
developed over many years into an interdependent and interconnected ecosystem comprising a
large variety of firms providing world-class services, products and advice. This ecosystem brings
significant benefits to financial institutions and to the corporations and households they serve.
Because of the interconnectedness of the activities and firms within this ecosystem, the effects of
the UK’s exit from the EU could be felt more widely than simply in business transacted directly with
EU clients.
Our analysis suggests that, at one end of the spectrum, an exit from the EU that puts the UK outside
the European Economic Area (EEA), but otherwise delivers passporting and equivalence and allows
access to the Single Market on terms similar to those that UK-based firms currently have, will cause
some disruption to the current delivery model, but only a modest reduction in UK-based activity. We
estimate that revenues from EU-related activity would decline by ~£2BN (~2% of total international
and wholesale business), that 3-4,000 jobs could be at risk, and that tax revenues would fall by less
than £0.5BN per annum.
At the other end of the spectrum, in a scenario that sees the UK move to a third country4 status with
the EU without any regulatory equivalence, the impact could be more significant. Severe restrictions
could be placed on the EU-related business that can be transacted by UK-based firms. In this lowest
access scenario, where the UK’s relationship with the EU rests largely on World Trade Organisation
(WTO) obligations, 40-50% of EU-related activity (approximately £18-20BN in revenue) and up to
an estimated 31-35,000 jobs could be at risk, along with approximately £3-5BN of tax revenues
per annum."