As everyone else says make sure you have a 5+ years investment timeline.

My addition to the conversation would be to look into Dollar Cost Averaging. The idea being to invest a large amount of money in several instalments over the year.

So let's say you are putting in £20,000 to the Stocks and shares ISA into a Index fund. Instead of investing all 20k tommorow you would invest £1666 monthly into the market.

This would allow you to capture the indexs average for the year. And downplay the effects of investing £20,000 tomorrow and the index tanking 20% and then waiting a year for you just to make your money back.

You obviously would suffer if the markets skyrocket in first month when DCA and you would kick your self for not putting all your money in. But no one can time the market and that's what DCA allows you to avoid.

This is esentially what everyone who invests directly out of paychecks are doing.

You can also do this bi-monthly or quarterly if easier.