Re: How about 20% tax?
Because the DST is a revenue tax. So it's on total revenue, not profits. If you were operating on a 4% profit margin, then a 2% DST effectively amounts to 50% of your profits (rather higher than the UK Corporation Tax bands of 19-25%). Now I doubt Amazon's margins are as razor-thin as 4%, but that 2% of UK revenue is going to be a much larger percentage of their UK profits.
Of course they'll calculate DST as an expense, reducing the taxable profits, so HMRC loses a bit of CT, but on balance wins.
The concept of a revenue tax is pretty controversial because you could be taxing unprofitable companies as well (e.g. startups, or companies doing high levels of R&D/investment), which is why this is quite specifically on firms generating over half a billion in global revenue and at least £25m from UK users - we can presume they're profitable at that scale (and if they're not, then that's very much their problem, we're not crushing small-scale domestic innovation).
The DST was necessary because all these firms were reporting huge revenues but contriving to declare improbably low profits from that enormous turnover, thereby avoiding Corp Tax. We've also seen in this week's budget a move to cut business rates on hospitality businesses (i.e. local, physical premises - pubs, restaurants, entertainment, etc) whilst increasing rates on big-box warehouses which employ relatively few people per sq.m and are disproportionately owned/operated by the sorts of multi-nationals who prefer to pick and choose where and how much tax they will pay.
But you can't run a 20% revenue tax. Even the likes of Google are unlikely to be have profit margins significantly in excess of 20%. We could probably push to a French 3% or maybe even 4% pretty safely though.