wwrondo
I realise some of these words could cause offense to some (while others don't care, see it in a funny way etc) and hence the apology and agreement not to use them. Your "I" Brexiter words would have the same negative qualities though.
Back to the topic.
If you google UK Financial services GVA you will get numbers like 8% or so. I remember seeing 9% at some point but I round it up to 10% - such measurements are not precise anyway.
As you said, the City is not uniform and so some areas will be affected more while other's less. We live in a world where call centres are in India Phillipines and a CEO issues orders from his private jet crossing the Atlantic, the real works are then performed where the competencies are.
Investments euros go where the profits are although a marketing/relationship office will need to be in the client city as it is now. Despite a single market, many banks still choose to position their offices close to clients - the idea that a single market can be fully accessed from a office in Dublin or London has never been a realistic one. Once the businesses are obtained and the real work begins, the works are sent to where the competencies are - NY, London, Shanghai, Kuala Lumpur, Manila etc. London is now both relatively cheap and less regulated. Works will go there.
Some Euro Clearing jobs will have to be in Dublin (boring city) but I accept some jobs will be affected.
There are only 2 global financial centres that is NY and London with HK/SG coming close by, and then Dubai, Shanghai, Frankfurt, Tokyo. Another pattern other than the regional gateway is that these are all English law jurisdictions which follows the Anglo-Saxons capitalism.
The more than half of London that is currently serving RoW and UK Domestic needs will be rather unaffected.
The world still trades in regional block but the trend is to move away from that. Shipping is awash with capacities. UK trade with EU as a % of total trade has been falling steadily.
I am afraid I have opposite view to you regarding the EU block. It is a block riddle with internal conflicts and cultural differences which mix business with politics and will likely to be more of a hindrance than 'help'.
Does China really has our interest at heart and want the EU to balance against her own rise? I am not convinced.