I'm not familiar with the UK market so I don't know if there are no-load mutual funds there. However, there is nothing stopping you from investing in the US market.
If you don't know what a mutual fund is, it's a large pool of money belonging to individual investors that is professionally managed by a fund manager. The money is invested in the stocks of many companies, if it is a stock fund. There can be bond funds, there can be commodities funds, there can be funds that invest in a mix. The purpose is to spread the risk. You don't too much of your overall money invested in any one thing - something too risky can seriously hurt you, something to safe can seriously hold you back.
Next thing to know is the 'Rule of 72'. This rule states that if you know one of two things, you can find out the third. It is about doubling your money. If you know the rate of return on investment, you can find out how long it will take to double your money. If all you know is how fast you want to double your money, you can find out the rate of return you need to do it.
You simply divide the number 72 by the number you have and that gives you your answer.
For you to turn £600,000 into at least £5 million, you need to double your money a little over three times:
600 to 1.2
1.2 to 2.4
2.4 to 4.8
A reasonable rate of return is 6% annually. You can do better some years, even much better, but not consistently. There will be years when you actually lose money. At 6%, divided into 72, your money will double in 12 years. At a consistent 6%, you would have 4.8 million in 36 years and 5 million in another year.
If you could get a consistent 8%, your money would double every 9 years and you would reach 5 million in 28 years.
If you guess right ALL OF THE TIME on the NEXT BIG THING (the next Amazon, Google etc) with ALL OF YOUR MONEY, you might get to 5 million in 5-10 years. You might also end up broke. NOBODY has a 100% 'guessed right' rate, not even Warren Buffet and others like him. You aren't going to do it.
So stick to no-load funds. Avoid individual stocks. The stocks you like will probably be owned by the funds you are in, in small enough amounts to limit risk.
There are also load funds out there. These are sold by brokers who collect a sales commission out of your money. You invest 10 thousand with a 5% load, only 9,500 of your money gets invested for you. No-load funds do not have these sales commissions. They do have management fees, and you need to pay attention to that. The best funds not only have a track record of performance but have fees well below 1% per year on the total value what you have in the fund.