How to assign a unique ID for each company in a list

Hi, I have a pretty complex task here. To make it simple to explain, I attached a sample file here:

I want to select those unique companies from the list by creating a unique ID. That is, if a company is listed in multiple stock exchanges, then only the stock ticker appear first will be selected. If a stock is listed in London, then London will be prioritized.

Column E shows the results that I want to achieve with Excel formulae. Hope this explains what I hope to achieve.

Thanks in advance.