Please re-think the FTSE 100, the performance is diabolical.
Read: Investing Demystified https://www.amazon.co.uk/gp/product/B071WQ1L81?ref=dbs_p2d_P_W_kindle_available_T1
Think global, the UK accounts for ~4% of the global stock market, don’t put all bets in one country etc :)
Vanguard FTSE Global All cap Fund, VWRL are both good starting points :).
I'll offer some of my favourites for your consideration :-D
* Investing Demystified - Lars Kroijer
* https://www.amazon.co.uk/gp/product/B071WQ1L81
* Introduction to Investing in Index Funds and Etfs - Mário Nzualo
* https://indexfundinvestor.eu/introduction-to-investing-book/
* Understanding Stocks 2E - Michael Sincere
* https://www.amazon.co.uk/gp/product/B00HQ3EUWI
* Understanding Options 2E - Michael Sincere
* https://www.amazon.co.uk/gp/product/B00GWSXX8U