Any UK resident can buy shares. Looking at the ETrade site the problem appears to be that the Etrade account is specifically a UK trading account and specifically is used to "Trade all UK and Irish dual-listed stocks and shares, Bonds, Covered Warrant, ETCs and ETFs".
You didn't say whether what timeframes you were going to be holding these US shares for, but the tone of your message suggests longer term. You need to find another broker that will deal in US shares.
Just as one example look at Charles Schwab, U.K., Limited, but there are many more UK and US brokers who will do this - use Google to find them or wait for suggestions here.
If you are not daytrading then you will not need to have the minimum of $25,000 for a US account.
If you open a US account then you will need to fill in a form so that you are exempt from US tax on your investment.
You could also buy US equities via a UK broker, but I think the charges might be a little more, but if you are holding for the longer term then this would be insignificant.
