I dont see why. The worlds 5th largest economy and London being the worlds largest financial market.
Because as the 5th largest economy (or 9th depending on how you measure it), we have shot ourselves in the foot. Just about every study that has looked into this has stated that we will worse off.
Yes, we are a major financial market, and many major financial institutions are set up in London because of the access to the EU. Its no mystery why many are either locating now, or making plans to do so. This is a loss to the UK, and is our own making.
The following quote is from here:
https://www.ifs.org.uk/publications/10303
"A lot of cross-border trade in legal, business and financial services is trade between companies; firms in London providing services to other firms in Paris, Rome, wherever.
Indeed, more than half of the UK’s imports of goods and services from the European Union are of this intermediate type. In the other direction, the numbers are even more striking. Seventy per cent of our exports to the EU are of intermediate goods and services; that is they are exported for use by companies, not for purchase by consumers."
-The link is a good read into why we are better off in the EU, and why trade deals are complex and of huge benefit. Something we will lose with Brexit.
We will lose not just jobs unnecessarily, but tax revenue, as that sector is our biggest.
Oh, and we won't be the 5th biggest economy for much longer, given that India is growing at a faster rate than us, even without Brexit.