at the beginning, they "charged" $1 (or £3 as you said), but this "fee" was often just waived. You never really had to pay it to use whatsapp. The money was there as a form of advertising, to differentiate whatsapp from the others - because by making it seem more premium via attaching a price, it makes the people using it feel more superior and thus the platform more easily propagates; and it's also why they "secretly" let you use it for free if you refused to pay.