How can I ensure that my savings keep up with inflation, even after any tax due is taken off the returns?
If your primary concern is inflation, you can buy an Inflation-index bond. This is a widely available bond that many countries offer. In the UK, this is referred to as the "Inflation-linked Gilt", issued by the UK Debt Management Office. These bonds will value the investment in relation to a certain inflation index. In the case of the Inflation-linked Gilt, they use the Retail Prices Index.
is an exchange traded fund (ETF) that aims to track the performance of the Barclays World Government Inflation-Linked Bond Index as closely as possible.
You still need to take into account commissions, among other factors, and understand that any investment comes with risk. Note that this ETF aims to track the performance of an inflation-linked bond index. It may not succeed all the time. This isn't a fool-proof strategy, unfortunately, because investing is always a trade-off between risk and return. By excepting a higher return, i.e., one that can keep up with inflation, you do bear a higher level of risk.
Vanguard.co.uk has good general information about inflation-indexed instruments and funds too.