You guys are making this more complicated than it already is... 
Doesn't help that I suck at explaining things tbf. 
The phone has been chosen. My current tariff is fine.
The phone is cheaper to buy with either an O2 or EE SIM than with no SIM at all so makes sense to buy with a SIM. I wanted to know which SIM would last longest on £10 before reverting to current SIM. It seems irrelevant now though as both only last 30 days, after which it'll get binned for my original, despite not using much of the 'bundle' that comes with all PAYG SIMs these days. So it turns out it doesn't matter which SIM it comes with.
Also, the fact I don't use the phone as a phone, doesn't mean I won't need a decent one, I'm essentially after a camera, with internet surfing and a calculator that fits in my pocket. The fact it makes phone calls is largely irrelevant. As I say, have chosen one now anyway. 
Doesn't help that I suck at explaining things tbf. The phone has been chosen. My current tariff is fine.
The phone is cheaper to buy with either an O2 or EE SIM than with no SIM at all so makes sense to buy with a SIM. I wanted to know which SIM would last longest on £10 before reverting to current SIM. It seems irrelevant now though as both only last 30 days, after which it'll get binned for my original, despite not using much of the 'bundle' that comes with all PAYG SIMs these days. So it turns out it doesn't matter which SIM it comes with.
Also, the fact I don't use the phone as a phone, doesn't mean I won't need a decent one, I'm essentially after a camera, with internet surfing and a calculator that fits in my pocket. The fact it makes phone calls is largely irrelevant.
As I say, have chosen one now anyway.