I just called Sky Mobile to cancel my sim plans as my offer period had ended, and I was offered the 8GB plan for just £5 a month!
They had to move my plans to the 3GB for £6 a month plan initially to then activate the half price offer, before then immediately changing me to the 8GB plan, which is currently on offer for £10 a month. Combining with the half price offer, this takes it to £5 a month which is fantastic!
Potentially existing customers / retentions offer only, but worth a call!
Good luck, hope I can help someone.