Don't know if this has come up before, anyway ....
I registered a new Asda mob sim card today by phone with their help team. They told me that if topping-up by phone using a credit/debit card, the minimum spend is £1. Thats right, one pound. Voice calls are then 10p a minute, 4p texts. Minimum one use per 6 months. Data allowance unknown but (I would think) close to non-existent. None of this shows on their website.
So; no anonymity but it has to be the rock-bottom rate of anywhere. Ideal for the infrequent user.