Attend Saturday morning Yoga classes for £20 per month. If you attend weekly this means your Saturday Yoga class costs you just £5, a 40% saving on the drop-in rate of £8. Your membership will automatically renew every month, so you don't have to worry about organising payment at all after initial sign-up.
If you prefer to come less regularly to Yoga, take a look at the Saturday 4 class pass which is £24 and allows you to attend four classes within 3 months (£6 per session).