“Top up” and “buy a Pay As You Go Plan” are not the same thing. A plain top-up adds credit to your SIM at EE’s standard PAYG rates (good for emergency use). A Pay As You Go Plan converts £10 or more into a 30-day allowance with unlimited UK minutes, unlimited texts and a chunk of data.
If you only need a few quid in case of emergencies, top up. If you plan to use the SIM regularly, go straight for the £10, £15 or £20 Pay As You Go Plan online or in the My EE app, it works out far cheaper per minute and per GB.