The problem with this question is they never mentioned or specified who is "they".

Especially in UK where The Rich that dont belongs to UK in the first place will of course move and leave. Those who are born and raised are highly unlikely to move.

And perhaps most importantly the question completely ignore the real problem. There isn't a lack of Tax revenue, but the government incompetence to allocate and use it wisely. To raise tax is just saying what we are doing now is absolutely correct and we will continue to do so.