Interactive Investor is probably one of the best brokers out there.
Regular investments are free.
However, check the fees carefully, because if you have a very small amount then other options can be cheaper.
Don't buy funds: with funds you quite never know at what price you are buying or selling, because orders take a few days to settle. Buy ETF. use a website like justetf to educate yourself on how ETFs work and to find ETFs to invest in.
If you want to go for global trackers, eg on the MSCI World, you'll have plenty of options.
Remember that ISAs cannot be joint accounts, but in one name only,
The main advice is very simple:
keep an emergency fund invested in super-safe things like saving accounts, cash ISAs, or short-term gilts. If you need money for an emergency (the car breaks down, you lose your job etc) you want these money to be available. This is the pot that maybe doesn't beat inflation but at least doesn't go down in nominal value. If you need access to this emergency fund, you cannot afford the risk it is maybe worth 10-15% less because you are unlucky and need it just when the markets crashed
anything that you realistically don't need for a period of 5-7 years, invest away in equities. Over these time periods, equities have tended to outperform safer investments. There is of course no ironclad guarantee they will continue to do so in the future, but there is no safe investment which yield a lot. You can have safe which yields a little, or riskier. Safe which yields a lot doesn't exist!
The idea is that your investment will go up and down in the short term, but, over 5-7 years, it is very likely to go up