I have a 12 month SIM only contact with O2 that gives me 10GB of data & unlimited texts and minutes for £18, I checked if I can up my data allowance and it turns out that I can, for £2 more I can get double my data. Hope someone else will be able to benefit from this, especially if you're on like a 4GB for 18.75 as is their current offer. £20 for 20GB, £25 for 25GB, £30 for 30GB these tiers are offered to me. This does not renew your contract for another 12 months just so you know.
O2 SIM Only - change tariff to 20GB for £20, 25GB for £25 & 30GB for £30 w/ Unl minutes & texts (12 month contract)