Meditrina - to answer your specific question:
Last year, total tax revenue collected by HMRC was just a shade under £409 billion. That's income tax, VAT, NIC, Corporation Tax and all the other odds and sods.
My best guess estimate for how much tax HSBC pays across to HMRC is around £10 billion. That's made up of CT, PAYE withheld from employees, employer and employee NIC and irrecoverable VAT.
So around 2.5% of all tax take comes directly from HSBC or from its employees.
If HSBC were to shift to Hong Kong then its UK retail banking operations would stay here, of course, and those would still be tax paying. So you could argue that the £10 billion figure is an overestimate. That said, there would be a knock-on effect for all the other businesses that're reliant on HSBC, so I reckon that around £5bn to £8bn in lost tax is about the right figure.
£8 billion in lost tax is roughly equivalent to increasing the basic rate of income tax by 5p, or 2.5% increase in VAT.
I very much doubt that it would be possible to collect £8 billion per annum from the rich. Have you ever heard of the Laffer Curve? It's the idea that when you raise taxes you collect less that 1p of tax for every 1% rate rise, because the increased tax disincentivises people from making the effort to earn more. That's the reason why the Institute of Fiscal Studies has calculated that the 50% income tax rate will actually cost the country, rather than raise revenue.
So, in my view, if HSBC leaves the UK we would be screwed.
The problem is that the UK is used to thinking of itself as the only place worth doing business, and hasn't really woken up to the fact that the various pluses of being here (time zone, language, educated workforce, benign income tax regime, etc) are starting to be outweighed by the minuses (poor transport links, high house prices, red tape, somewhat unbusiness friendly corporation tax regime when compared to many other places, etc).
Hong Kong has a totally different basis of taxation, as well as a narrower tax base and a lower tax rate. I would be surprised if Hong Kong were to collect more than £1bn in tax from HSBC per year all in.
(BTW, it's my job to calculate these kind of figures for governments around the world. So I'm confident the numbers, although guestimates, are in the right ballpark.)