The thing is... this whole 'if you tax the mega rich more than they will just up and leave' is not based on actual evidence. It also doesn't take a great imagination to wonder why the theory gets thrown around, almost as if it is fact.
They did a study on every million $ earner in the USA and these people could literally just move to another state to pay less tax if they wanted to... they don't even have to go to another country. Yet the levels of inter state migration was lower than for poorer or middle class people. The reality is, that when you have that much money... place is actually more important than the extra $. If you are a millionaire, settled with a family and business... are you really going to up sticks and move just to pay a bit less tax?
I think when thinking of taxing the top 1% (or 0.1%) we have to consider the reason that it isn't already happening.
For every single political party of any persuasion, taxing the top 1% is an absolute no brainer. They represent a tiny minority of the votes and the money they could provide would win a large majority of the votes. It would be the easiest ticket to perpetual election winning if it could be done.
The problem is you can't tax them anymore than we currently are. It isn't just about them leaving, it's about them finding ways around paying more than they feel is fair.
The part you're not accounting for in your migration statement is that the top 0.1% particularly aren't paying tax according to the doctrines and rules you or I do. They are paying tax according to their own rules. If they want to pay less tax they wouldn't need to move to do so.
Essentially the cost to make them pay more in tax outweighs the tax the country will receive. Again this is obvious as there is every single incentive to tax them to a greater level. Any party who was able to shake them down for another £100 billion would win a landslide.
That's why the truth is that any party who talk about increasing taxes, that are already at an all time peacetime high in comparison to GDP, can only be talking about taxing two groups of people: middle earners and lower earners.
The reason political parties don't tax middle earners further is because they're the bulk of the voting base. Turkies do not vote for Xmas so for any party this won't happen.
We then are left with poor people. They vote to a far smaller degree than every other class and whilst they don't have much to give individually, collectively an increase such as the 5% on VAT, another 5p on fuel duties or 5% on council tax which disporportiontely hurts them is an easy winner. Poor people also use public services less as they die younger, go to higher subsidised education to a much lower rate etc
Likewise punitive corporation tax rates (if they weren't easily avoidable) merely push the price of the goods those companies supply. If we have 3 competing bread manufacturers all of whom need to make a £1b profit to keep their shareholders happy... What do you think occurs if the government take £500m of their profit? The price of bread increase and the money in the pockets of the poorest decreases.
Any vote for an increase in taxes is a vote for taxing poorer people disproportionately. That's why we should be voting for an abolition of these taxes.