UPDATED: Now £10/month!
£10/month - 12 month sim only contract includes unlimited minutes, texts and 15gb double speed data, and a JBL Flip 4 Portable Bluetooth Speaker (although I'm not convinced their RRP of £119.99 is realistic...) Based on an existing BT customer
Possible cash back through quidco (£75) brings the equivalent monthly cost to £3.75/month.