"By 1990, more than 50% of all Swedish trading had moved to London." - Wikipedia. But the Swedish tax was 10x higher and Sweden didn't have large stock markets.
Even more, the EU is taking the American approach to taxing there ("if you're American you're paying, regardless where you're living"):
Banks with headquarters in the EU have to pay the tax, even if the transactions are done in Somalia. Every transaction in European stocks is taxed, even if done in Afghanistan.
Well, then, the banks will move.
Doesn't help. As long as European stocks are being traded the transactions are taxed.
Expect the listings to move to NASDAQ. A bunch of European companies have primary listings there already.
Somalian banks tend not to have passporting access rights to the EU market.