As a tax adviser I think this is an unworkable idea. Apart from anything else it would be ridiculously complex to administer and clog up HMRC and the courts with cases about how it should be applied. It would also be very difficult to prevent avoidance. Just some of the complexities - can debts be deducted from wealth and, if so, is car finance, mortgage, student loans etc. included? What about a personal guarantee someone has given their business which is in debt? Do all assets need professional valuation? Are assets outside the UK included and, if so, how does wealth tax interact with the tax of other countries? What if a person gives away assets before the tax is applied? Would a person need to sell their business in order to pay this tax or should there be a relief for that, like for inheritance tax? I'm not saying it's impossible to do but it would be a nightmare. It's hard enough for HMRC to collect tax on income and they have had decades to work out a system for that.
It would also lead to people dumping their assets onto the market in order to pay the tax which would lead to house prices and potentially share prices crashing. You would need to allow payment by instalment over a long period to prevent this.
There are alternatives (like printing money) which get to essentially the same place without some of the advantages of a wealth tax. A government voucher scheme would slightly redistribute wealth depending on how it was done - if one person has £100 and one person has £1000 and you give both £500 the first person now has £600 and the second £1500 so they are more equal than they were before.
For those saying close loopholes or tax amazon and facebook, I wish people would specify what they mean by this. HMRC and the government (particularly the conservative government) have made huge efforts to crackdown on tax avoidance in the last decade and to a large extent these efforts have been successful. I could name a hundred pieces of legislation aimed at preventing avoidance or closing loopholes. In my practice I see very few clients who want to pursue the sort of dodgy schemes which used to be quite commonplace. Avoiding tax really isn't as simple as hiring an accountant and asking them to dream up some scheme for you and most midsize and large UK companies are now very conservative on tax issues because they are worried about reputational risk and challenge from HMRC. There is a new tax being planned to tax turnover of large tech companies and the UK is leading the way on this. Usually, when large multinationals aren't paying much UK tax it's because another country (e.g. the USA) has taxing rights and the UK doesn't have the right to tax under international law and treaties. I do think HMRC could be better funded to apply the laws they have at their disposal though and that that may increase tax revenue but nothing like as much as people expect.
I think the answer here is to print some money, do what we can to get the economy back on track by encouraging spending (like with the voucher scheme) and maybe tinker with tax rates later. Personally I think massively increasing inheritance tax and lowering the threshold for it would be the fairest thing to do to increase tax revenue but inheritance tax has always been politically unpopular and relatively easy to avoid so I doubt that will happen.