Can I change to a plan advertised on Money Saving Expert?

Can I change my plan to a plan advertised on Money Saving Expert?

It's for new or re-joining customers only.

Thanks. I need just a bit more than 5Gb, say 10, but my only option is 6x more data for double the cost. It’s a shame there’s nothing in between.

You can transfer out for a month or 2 and come back.