For tax free interest, invest in an ISA.
I would only tie in for a few years though, as interest rates may rise in the next few years and then you'll be left tied in ......
I know rates are a bit naff at the mo, but they're naff everywhere ..... if you want higher interest rates, you'll prob have to commit to a longer term, or you'll have to consider riskier options, such as shares, etc.
My opinion: invest in an ISA for no more than 3 years and then when the 3 years are up, weigh up what's on offer and invest in the best option available ......