It will to a certain extent depend on your attitude to risk and the length of time you are willing to hold the funds for.
If you want to retain immediate access to funds, then interest bearing current accounts, savings accounts and/or a cash isa would likely be the be the optimum. However, instant access savings rates are generally below inflation and so effectively you would be losing money.
If are able to lock money away for a short period (a few months to a few years years), Notice-based and fixed term accounts would give slightly higher returns, but may still struggle to keep pace with inflation unless locked away for longer durations (around 5 years). Cycling money from interest paying current accounts into higher paying regular savers would give an above inflation return on a portion of your savings.
As you start moving towards a longer timescale (10 years plus) then funds or trackers through an investment ISA open up as an option. You should consider being more conservative with your portfolio towards the shorter timescales. Don't try and time the markets, and don't worry about short term drops or celebrate spikes.
If you are uncertain of if/when you will want access to the funds, then a blend of the above may be appropriate, allowing you access to some cash in an emergency should you need it, a larger pool of cash for planned expenditure and the remainder in long term investments. Keep reinvesting the interest to benefit from compounding, and drip feed in more savings as you can afford it, and you should see your investment grow in the long term. However, it will likely take a number of years before it grows significantly enough to give you a meaningful side-income.
Other options out there include credit unions, premium bonds (government backed savings where you effectively swap interest for a raffle, with prizes up to a million pounds) and peer to peer savings. You could set up a business, or put a deposit on a property and rent it out. There's matched betting and arbing, but don't try that if you have an addictive personality or are otherwise prone to gambling. These all come with varying degrees of risk and potential reward.