AS above, unless interested in trading individual shares then either a fund or tracker. For direct online investing The Vanguard funds are probably the lowest cost trackers out there. I put 10K in the LifeStrategy 80 in Nov 2018. Up 17% at the moment.
LifeStrategy® 80% Equity Fund – Accumulation
https://www.vanguardinvestor.co.uk/
A fund you can invest in directly online is Fundsmith. Not a tracker. THe philosophy is invest in a small number of good compaanies and hold. They have outperformed tracker funds over the last 10 years. I have had cash in there since 2010.
https://www.fundsmith.co.uk/fund-factsheet
Either way its good practise to split savings among a few places. Even trackers can be diversified by choosing funds with different geographical splits etc.