James Dacombe was only 17 when he left school and started a brain-scanning company. Now aged 25, his new start-up is taking on one of the world’s most valuable companies.

The 25-year-old's company has tripled its valuation since it raised $220m in February this year.

A London startup two years old has just become one of Britain’s boldest bets against Nvidia. Olix has raised $312m and tripled its valuation to $3.3bn in six months. Its founder,…