buying a new iPhone

[beachbuoys]

I'm on pay-as-you-go with O2, using a very old 6SE iPhone.

I want to get a new iPhone and stay on O2 pay-as-you-go.

O2 don't seem to want to deal with this anymore as far as new iPhones are concerned, so what are my options, how do I find out how to get a new iPhone and deal with sim card etc. 

Complete novice here. If you could point me to a website that would greatly help.

[MI5]

@beachbuoys 

O2 have shifted all PAYG phone sales to giffgaff now.

https://www.giffgaff.com/mobile-phones?brands=apple

You can use your current sim card in the new phone or get a new blank sim from O2 and do a sim swap Guide: Sim Swap: a mini guide 2017 update 

[beachbuoys]

WOW!!! that was a quick response. I'll check out those links and see where it takes me...but thank you for  even responding.

[MI5]

You're welcome @beachbuoys 

I understand that some O2 shops still sell a limited choice of PAYG phones if you prefer to go in store or you are free to buy a sim free phone from anywhere you feel comfortable from doing so.

Amazon or CEX are safe choices but I would avoid eBay or Facebook marketplace.