O2 has recently  launched Classic Pay As You Go . With a simple flat rate of just 5p a text, 5p a minute ( 01/02/03 and Standard UK Mobile Numbers) and  5p a mb.

Classic Pay As You Go is useful for those not wanting bundles, inclusive minutes or minimum top up requirements.

Minimum top up £10.

Existing O2 customers are  able to transfer to the new tariff by contacting 02.

International Bolt Ons are available only  on Classic Pay As You Go.

You need to carry out a chargeable activity at least once in any 6 month period to retain your credit.

https://www.o2.co.uk/shop/sim-cards/pay-as-you-go/#simtype=classicpayasyougo