Well the comment about Macquarie is there for emotive reasons - who are those Australian newspapers, what are the allegations, why is a company focusing on profits a bad thing, what is appropriate tax minimisation etc? Perhaps the non-UK media would now say that about Thames Water purely off the back of the Guardian article? So I suggest we take that phrase out of the quote and focus on the rest.
What we appear to have is an idea that Thames Water is owned by some non-UK investors, via a structure that includes some Cayman entities. Is that bad?
The way that an owner can extract money out of a company are threefold:
1) By paying for some services that the owners are providing.
2) By paying interest on a loan.
3) As a dividend.
There is no suggestion that Thames Water is paying money to the owners for any kind of consulting services.
The payment of a dividend does not give rise to a tax deduction in the paying company, i.e. Thames Water do not get tax relief by paying dividends to the Cayman company. Further, a UK company does not pay tax on the dividends it receives. So on this dividend point, there is no tax avoidance or evasion going on.
Paying interest does give rise to a tax deduction, however the UK rules do impose some limits on this, so you can't saddle a UK company with a load of debt and strip all of the profits out that way. The rules are being further tightened to reduce the impact of this.
Payment of interest to a Cayman company would often have a 20% withholding tax at source. In practice, this can nearly always be mitigated. So in that respect, there probably is some tax advantage in having interest paid to a Cayman company, but only insofar as the investors are concerned. Thames Water would prefer not to pay interest out to start with, i.e. the tax on the extra profits would be lower than the payment of interest. Most UK businesses will have some form of borrowings in place to fund expansion, working capital etc, and get a tax deduction for this. So again, it's not a UK tax point but a matter for the investors' tax authorities as to whether they should pay tax on the interest income received.
So far, nothing dodgy from a tax perspective.
Why Cayman? We shouldn't be concerned that there is a Cayman company owning or lending money to Thames Water - as outlined above, that gives no tax advantage to Thames Water. It's a non-point.
As the Guardian have cited there, the investors come from a number of countries and are made up of investment funds (themselves, attracting investors from around the world). Those various investors will want to come together in a place that gives them the right sort of legal and regulatory protections they need, which Cayman Islands offers. What about tax? Yes, Cayman Islands has no domestic taxation so we instantly think its bad. Well actually, UK tax laws exempt most UK funds from tax anyway, so the UK is already a tax haven in that respect.
I know I'm being a bit one-sided in all of this but once you scratch the surface a bit, you'll see that a lot of what is going on here is not as bad as it is made out to be. I really can't see anything sinister going on.
EDIT: To clarify, Thames Water is a UK company. It's ownership does not change that. It is fully within the scope of UK tax, so the point I made about leasing assets is perfectly valid.
. I recognise that I am more towards the blue corner than you when it comes to capital markets (I'm surprisingly liberal-socialist in may other aspects, not true-blue), and so we do have a different viewpoint as to what we "want" the answer to be here, but I would like to think I have tried to explain things accurately and objectively in terms of how the tax system works. 
