I understand that these strategies are not country-specific, but not sure if Vanguard is accessible to non-US citizens, and there might be tax implications in different countries. That's why I'm asking.
[0] https://www.vanguardinvestor.co.uk/home
[1] http://www.scotsman.com/business/markets-economy/bill-jamies...
They have mentioned that Vanguard will be looking to give access to Europe in 2018.
[Edit] - Removed Financial Times link as they have a paywall.
For tax specific stuff you should look at your regional sites eg
https://global.vanguard.com/portal/site/home https://www.ishares.com/us/ishares-global
Vanguard operate mutual funds in the UK priced and traded in GBP, through their UK subsidiary. They're available through most major trading platforms. The one thing to note about Vanguard's UK funds is that the management fees are higher than their US equivalents (but still one of the cheapest in the UK market).
As others have said, you can also just buy ETF versions of most of their funds, which are regular shares traded on the stock market. In that case you're buying in USD.
As far as tax is concerned it's treated just like any other investment (and if you're tax resident in the UK and still have an ISA allowance you can just put it in there and negate the tax entirely).
Make sure to stick it in an ISA account if possible to keep it tax free.
In order to get average returns, the key is simply to own a tiny fraction of the entire market, whatever "market" means to you. The focus has long been on the S&P simply because it has historically done a pretty good job of representing the US equities market, it is market-cap weighted (unlike the Dow), and the earliest index funds tracked the S&P.
It's my expectation, stated without proof, that it should be easy to find a comparable index in Europe, if you want to match the average European return.
Non index funds perform the same as index funds (and research indicates you can differentiate the ones that outperform a priori with any method). But those funds have higher fees.
https://www.charles-stanley-direct.co.uk/ViewFund?Sedol=B41X...
http://www.investopedia.com/articles/mutualfund/05/etfindexf...
I'm from Europe and I'm holding couple of Blackrock ETFs.
Here's handy site for screening EU based ETFs (no affiliation).
Souverän investieren mit Indexfonds und ETFs: Wie Privatanleger das Spiel gegen die Finanzbranche gewinne
by Gerd Kommer
You can also google for Kommer Portfolio. It is, IMHO, very well balanced (asset classes, geography etc.).