In my opinion, the UK stock markets are the best and safest in the world to invest in particularly now we are out of the financially unstable and deranged EU.
The FTSE-350, comprises of the top 350 companies floating on the UK's stock exchanges and HL appear to recognise the risk to investors of companies outside of these parameters, say, the very higher risk AIM shares (and the associated liquidity issues).
Personally, if I didn't understand the stock market I would say stick with the FTSE-250 to FTSE-350 shares to reduce risk. These are the top companies on the UK financial markets, therefore, the FTSE-350 gives you access to invest in the UK's top 350 companies.
Basic understanding of compounding interest is important to long term investing.
Albert Einstein quoted the following “Compound interest is the eighth wonder of the world. He who understands it, earns it…he who doesn't… pays it.
The more safer places to invest are compounding interest in a bank or better, good interest paying building societies, chances in premium bonds and decent Investment Trust funds. You can earn lot's more individually investing in shares but you have to know what you are doing and understand the significant associated risk involved.