You could play on the slots and withdraw any winnings that you make above your original £20. You may get a big slots win if you do this.
Play roulette in the Vegas casino (Not the live version) and put £20 on either red or black (Or odd or even). If you win, you have just doubled your money and you can withdraw your profits. If you lose, then you'll get your £20 back as a refund that you can withdraw. If you are feeling really lucky then you could try to double your £40 to £80, knowing that if you lose, you still get £20 back.