How will London's role as a financial center be affected after the Brexit and what are possible alternatives? I looked around on this site and couldn't find anything close to what I'm asking on Economics SE, so I hope it is the right place to ask. I am interested in but have no background in economy, so all my assumptions are based on what I have read in European newspapers over the past weeks and months.

Before the Brexit referendum one argument of the "remain" supporters was that London as a whole profited very much from Britain's EU membership. With the free flux of people and money within the EU London could establish/defend its role as an international financial center. Many banks chose London as their seat as they could have access to the EU from an international and English-speaking environment. It was also argued that London acts as a bridge between Europe and North America.

After the Brexit vote I read that many financial institutes consider leaving (or preparing to leave) London in favor of some other places. I have no idea how a typical day of someone working in the financial area looks like, but from my position I do not really see a big impact if Britain leaves the EU. Several disadvantages already exist today, like varying Euro-Pound exchange rates or border controls. The only real reason I can think of is that there may be higher costs on financial transactions between the UK and the EU. Is that all? How would London's financial market be affected should the UK definitely leave the EU?

Also, where would these institutions go? The most frequently heard names are Frankfurt and Paris. Paris as the capital of the second-largest economy in Europe with strong presence of financial industry, and Frankfurt as the main financial center of Germany with important institutions like the European Central Bank. Arguments against are that in Paris it will be rather difficult to communicate in English in everyday life, and Frankfurt is a relatively small city (while I don't know how that alone would be a disadvantage). So what would be potential alternatives for a firm deciding to leave London?
London remained for long as a key financial centre for Europe due to several reasons:


Historic: several universities and authors dedicated to the study of wealth (Smith, Ricardo, Malthus, Keynes
Combination of privileged international positions with US (language + colonization) and commonwealth countries, as well as an average sized internal market
Looser regulation than most countries in EU, providing lower barriers to entrepreneurship
Fairly efficient justice system
Management and investor attitude: social class or country of origin is less of an obstacle than in France, investors not as conservative as in Germany


Comparing these factors to other large cities in Europe, it will be hard to match it. 

In Germany, Munich or Berlin could be more attractive for investors than Frankfurt. Paris would be the favorite in France, but regulation doesn't attract investors nor entrepreneurs - so Amsterdam or Luxemburg would be much more inviting in this regard.