First buy a pay as you go EE simcard then call 150 to upgrade to a pay monthly sim plan. They will offer EE Sim Only 160GB 5G Unlimited minutes and texts for £14 per month. The contract is 24 months.
They will ask you to top up for £5 which is deducted from your first bill.
This is the cheapest deal I've seen and no need to claim cashback.