Re: Erm
The biggest source of avoidance - and loophole is probably the wrong word for it - is the entire way that international taxation works. For multi-national corporations there's a world-wide competitive market in corporation tax.
A country with a relatively small real economy can offer a better price in the form of lower corporation tax if, by doing so, it can get a few large multi-nationals to be head-quartered there for tax purposes. By their size, even at a lower tax rate, they will contribute sufficient to the smaller economy and, as a bonus, local businesses are also enjoying the lower tax which can help them be more competitive selling abroad.
In short, if you're a multi-national you can arranged to be taxed in Ireland on profits made on the business conducted in the UK. It's perfectly legal, it's not a result of some quirk of UK tax legislation but of the freedom to do that and the disparity in tax rates. Sorting it out would require international agreements and those smaller nations who benefit are in no hurry to agree.
Not that I'm explaining this to you, Codejunky, as I'm sure you're as well aware of that as I, but there's always an element of the commentariat which isn't.