To go back to the OP, who had a notional £10k.
£10k of lottery tickets, with all prizes less than a jackpot reinvested, gives odds of winning the jackpot of 1:6,694
£10k of premium bonds held for a year, and other winnings reinvested, gives a chance of winning £1m of 1:516,555 but of course you’d still have the £10k+winnings for more attempts next year if you didn’t win.
£10k on roulette, if you just play red/black until you pass £1m or lose it all gives odds of 1:5,352.