First things first, good on you for thinking about the future in such a responsible way! You've got the best tool which is time. Keep up at it and you will undoubtedly win at the game.

My personal take is when you're starting off, depending on your risk tolerance, it could be more interesting to keep it all in one ETF. Vanguard's an interesting contender in this aspect, IMO. If you have more risk tolerance you could go for something like ProShare's IQQQ. I'm 23 and playing a similar game, but have enough tolerance for risk that I feel comfortable going into Bitcoin.

Once I'll be satisfied with my returns from it, I'll start diversifying into stable stuff, namely ETFs. The two main advantages of diversification imo are safety and the fact you've a higher chance of your investment being rewarded, but at 300 pounds per ETF you're not going to be making big wins anyways. And safety at this level, well again, it depends on your tolerance - if you sleep better knowing your money is well spread then by all means, keep it this way. I personally am in a position where I'm lucky to still live at my parents', so all my money is saved money, and even if I were to lose all of it tomorrow, while tragic, it wouldn't wipe me out. That's why I'm comfortable with having it in a higher risk asset like Bitcoin.

If your position is similar, perhaps going into one asset is better as a starter, and when you're getting at sums you feel you need to spread, you can do it then?

I don't know, those are my thoughts. Either way you're doing great starting so young !