If you increase taxes even more there will be a mass exodus of high earners.
To where exactly? France, Germany, Italy, Greece, Portugal, Sweden, Netherlands, Denmark, Belgium, Switzerland, Australia, Japan, Canada, Ireland, all have higher top tax rates than the UK. Spain, Poland, Estonia etc do have lower rates but strangley enough we don't see a mass exodus as things stand (they could move to the Czech Republic right now and save 30% don't).
Maybe you're thinking they'll all move to America? But the US have a pretty strict immigration policy and while the high earner might get in, if they are approved for a work visa, there's a good chance their families won't qualify. And once you combine federal, state, local, property and sales taxes with things like the need for private medical care I'm not sure you'd be all that much better off.