With my £20, I would filter a litre of tap water into a second-hand Volvic bottle with a drink-through lid (cost: nothing), brew up some Waitrose Mocha Java and pour it into my trusty flask, make myself a delicious cheese-and-salad sandwich, buy the papers (cost £2) and stroll up the canal from my flat in Maida Vale to Regent's Park, where I would settle near the boating lake for an afternoon of reading and relaxation.