Best way for non-taxpayer to invest?

TGS_2

Which is the best way for my wife to invest in the stockmarket? She is a non-taxpayer and will be investing £250 each month for at least 10 years. This monthly amount may increase as time goes on. She wants to be able to get a better return than a building society and is aware of the risks with the stock market. I was thinking about an ISA but realise that it may not be the most tax efficient way for a non-taxpayer to invest.