o2 sim through comparison site?

savergrant

Hi. You request a pac by texting pac to 65075 from your current sim. You will get a reply in a minute. You can then give it to another provider any time within 30 days.
Your contract will rollover. On some contracts you will keep paying the same monthly rate, others give an automatic discount once the cost of the phone is paid off. Once your contract ends means once you are free to leave without penalty.
If you request a pac before being free to leave they may tell you there will be a charge when you leave. This is calculated based on you using the pac immediately so as long as you delay giving the pac you will only be charged for service until the day of transfer. Do not cancel payments.
It is usually a good idea to order and test a new network before porting your number, that does mean a slight overlap where you are paying for two services but is often worth it for peace of mind.

savergrant

Finally on many networks you can request a sim and then purchase a deal for it once it arrives. Some sims can receive texts and calls without having been topped up or a plan activated. Others will not work until you have signed up to a plan.
How much data do (you think) you need?
Other providers piggyback on ee's signal so give similar signal strength, 1pmobile, rwg mobile, nowmobile, and the very much budget end lyca who switched all their customers from o2 to ee without warning recently.

eDicky

A PAC is obtained from the network you are leaving, that service is terminated when the PAC is used.

https://www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/costs-and-billing/switching/switching-mobile-phone-provider