If you are a US citizen living in the UK it's generally a bad idea to invest in mutual funds as the US will get you under PFIC rules and HMRC will get you under offshore non-reporting funds rules ie no capital gains exemption and you'll be taxed at your marginal income tax on all gains.
There's no easy way to comply with the IRS rules, but HMRC has published a list of overseas reporting funds. So you can invest in these and be taxed by HMRC like UK based funds. The full list is here
http://www.hmrc.gov.uk/collective/rep-funds.xlsand here are some of the Vanguard funds and corresponding EFTs that it includes. This is an excellent opportunity for US citizens living in the UK who want to invest some after tax money. With a UK cash ISA ladder and a few of these Vanguard funds you can create a very nice low cost investment portfolio that is also relatively easy to deal with from a tax perspective.
Vanguard S&P 500 Value Index Fund Vanguard S&P 500 Value Index Fund - ETF Shares
Vanguard S&P 500 Growth Index Fund Vanguard S&P 500 Growth Index Fund - ETF Shares
Vanguard S&P Mid-Cap 400 Index Fund Vanguard S&P Mid-Cap 400 Index Fund - ETF Shares
Vanguard S&P Mid-Cap 400 Value Index Fund Vanguard S&P Mid-Cap 400 Value Index Fund - ETF Shares
Vanguard S&P Mid-Cap 400 Growth Index Fund Vanguard S&P Mid-Cap 400 Growth Index Fund - ETF Shares
Vanguard S&P Small-Cap 600 Index Fund Vanguard S&P Small-Cap 600 Index Fund - ETF Shares
Vanguard S&P Small-Cap 600 Value Index Fund Vanguard S&P Small-Cap 600 Value Index Fund - ETF Shares
Vanguard S&P Small-Cap 600 Growth Index Fund Vanguard S&P Small-Cap 600 Growth Index Fund - ETF Shares
Vanguard Total Bond Market Index Fund Vanguard Total Bond Market ETF
Vanguard Short-Term Bond Index Fund Vanguard Short-Term Bond ETF
Vanguard Intermediate-Term Bond Index Fund Vanguard Intermediate-Term Bond ETF
Vanguard Long-Term Bond Index Fund Vanguard Long-Term Bond ETF
Vanguard Total Stock Market Index Fund Vanguard Total Stock Market ETF
Vanguard Extended Market Index Fund Vanguard Extended Market ETF
Vanguard Large-Cap Index Fund Vanguard Large-Cap ETF
Vanguard Mid-Cap Index Fund Vanguard Mid-Cap ETF
Vanguard Small-Cap Index Fund Vanguard Small-Cap ETF
Vanguard Value Index Fund Vanguard Value ETF
Vanguard Mid-Cap Value Index Fund Vanguard Mid-Cap Value ETF
Vanguard Small-Cap Value Index Fund Vanguard Small-Cap Value ETF
Vanguard Growth Index Fund Vanguard Growth ETF
Vanguard Mid-Cap Growth Index Fund Vanguard Mid-Cap Growth ETF
Vanguard Small-Cap Growth Index Fund Vanguard Small-Cap Growth ETF
Vanguard 500 Index FundVanguard S&P 500 ETF
Vanguard European Stock Index Fund Vanguard European Stock Index Fund ETF Shares
Vanguard Pacific Stock Index Fund Vanguard Pacific Stock Index Fund ETF Shares
Vanguard Emerging Markets Stock Index Fund Vanguard Emerging Markets Stock Index Fund ETF Shares
Vanguard FTSE All-World ex-US Index Fund Vanguard FTSE All-World ex-US Index Fund ETF Shares
Vanguard Total World Stock Index Fund Vanguard Total World Stock Index Fund ETF Shares
Vanguard FTSE All-World ex-US Small-Cap Index Fund Vanguard FTSE All-World ex-US Small-Cap Index Fund ETF Shares
Vanguard Global ex-U.S. Real Estate Index Fund Vanguard Global ex-U.S. Real Estate ETF