The high tax rate in the UK in fact discourages people to work once they earn to the point of the 40% threshold.
However, as the cost of living (especially rent) rises, it will soon gets to a point that the 40% threshold can only barely make the ends meet, creating massive social unrest.
Why does the UK has such a high tax rate compared to the Crown Dependencies and the Colonies?
Tax havens attract rich individuals to base themselves there, thus paying 20% on half a million of income, rather than 40/45% on that same income in the UK. Ideal for people who've retired or who earn enough to "commute" longer than average distances, i.e. lots of people live in the IOM and then fly to London to work when they have to (richer people often have more flexibility with when and where they work).
As for companies, again, they place their "head office" in a tax haven which is basically an administrative address - some offices (even in IOM) are the registered office for hundreds of limited companies, despite being small and only having a handful of staff! If I remember rightly, a limited company based in IOM pays a fixed annual charge to their parliament, rather than a percentage of profits.
So it's a numbers game. A disproportionate number of richer people and limited companies paying relatively small amounts, but in high numbers, compared to relatively small/cheap islands. Of course, "richer" people will probably also place less burden on local services, i.e. more likely to have private healthcare, private education for their children, pay for their own social care (probably already have household staff anyway), less likely to resort to crime or anti-social behaviour, etc.
Nigel Mansell didn't "move" to the IOM to travel on the steam trains and look at the Laxey Wheel! Lewis Hamiltion didn't buy his private jet via the IOM because he liked looking at castles and riding the horse drawn tram! IOM benefit from being a tax haven, and the residents (whether they're rich or not) benefit from the low tax, same as other tax havens.