OVIVO Mobile offers cheap-as-chips tariffs in exchange for five seconds of ads when you open your browser, and they’ve just increased the monthly allowance on the Freedom package (which carries a one-off charge of £15) to include 150 minutes, 200 texts, and 500MB of data.
After the one-off charge of £15, you pay £0PM and the allowance automatically refreshes. If you go over your allowance, you simply eat into that initial £15 of credit.
OVIVO Mobile uses the Vodafone network.
(Thanks to JonT at HUKD)