Depends on how much you use calls and texts. I have dual sim phone on 2 networks. One is 1pMobile…1p per minute, text or MB for a £10 topup valid 4 months. Other is iDmobile…8GB data at £8 per month. Both of these have data rollover. I use very little calls or texts and usually use free wi-fi wherever I go so 1pMobile is best for me.
Smarty.co.uk might interest you. Similar deal for £6.25, with the bonus you get money back on data you don’t use. The base price which you’ll always pay is £5 I believe.
Private message me if you’re interested for a referral link and you’ll get a month free. Plus a £10 Amazon voucher apparently.