I'm looking to lower my cost on my Pay Monthly SIM

[Daz_26]

I can get 120GB per month with Three.com for £18, whereas the lowest comparable tariff with O2 is £23.99 per month!!

[Enlli]

Use U-Switch 

https://www.uswitch.com/mobiles/compare/sim_only_deals/?inset-cookie-banner&utm_accountid=5303720174...

Accept and it takes you to O2 where you can choose upgrade 

[Dino1968]

Yep Uswitch is the way to go. I got a much cheaper deal when it came to extending my o2 contract after paying off my handset. Plus as I am with Virgin for broadband I got double data.

[Oxonian]

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@Daz_26 wrote:

I can get 120GB per month with Three.com for £18, whereas the lowest comparable tariff with O2 is £23.99 per month!!

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If you are happy with the O2 network @Daz_26, I would follow the advice of @Enlli and @Dino1968 and stay with O2. the Three network can be very fast in certain areas ; but it is notoriously "patchy" and you sometimes cannot get a signal at all in rural areas.

Should you still feel inclined to move, I suggest that you get a Three PAYG SIM and test it in the areas that you frequent before committing yourself. 👍