In this day of computerised transactions and online everything I dont really see why we need any financial 'centres'.
They have to operate within the legal system of a sovereign state. They therefore need a business address in that jurisdiction. It is possible for a major financial institution to have an address in a jurisdiction, eg the City of London, but to have its main offices in a different nation. It is now perfectly possible for the UK to attract EZ business which will not physically move, just change its place of registration. The UK would not benefit from more jobs in the UK but would benefit from tax on their activities. Because they would be working within English law and in sterling it is marginally easier to do business with UK companies, so there would be a small boost to UK business.
London is the biggest financial centre, generating around 1/10th of UK's GDP. Other UK finance brings it up to about 1/8th of GDP. For every pound directly attributable to finance there is around another pound in supporting industries. Around a quarter of the UK's GDP is directly or indirectly attributable to finance, basically to the City. We in the UK are absolutely dependent on the health of the City. The UK sinks or swims on this.
The risk for the UK is to lose influence by being outside the EZ. However The potential is to get EZ finance, and the trump card is precisely that the UK is outside of a failing currency and the absurd regulations of the EZ.
Cameron had to veto the new Treaty, or refuse to sign up, or whatever expression you want to use.