Honestly, look elsewhere.
I don't mean to be rude but the market is extremely competitive and has been for some time at both ends of the scale and there is a ton of choice available to customers.
I'm paying £5.01/mo for the airtime portion of my contract, (once the cost of the handset and music subscription is deducted), and that gives me unlimited UK minutes, unlimited UK SMS, 500 minutes to EU countries and 80GB of data, (and I can use it all in 77 countries for free and that includes in country calls and SMS too).
And there are a ton of other tariffs between the giffgaff £5 package and my £5 package from all manner of different providers.
You can always come back to giffgaff when your usage fits the packages in the future.
There is zero point in having loyalty for a network, (any of them), be mercenary and go for the one which gives you the best bang for your buck based on your usage and your needs.
I've been through giffgaff, PlusNET/Life, Tesco, Three and Vodafone in the last 6 years, I've had a better deal for me, (based on cost/usage), with every single change I've made.
I'm currently very happy with my provider so I signed a 2yr contract recently as I don't think that's gonna change, but when my contract is up I wouldn't hesitate to move if my current provider wasn't up to satisfying my needs any longer or if someone else was offering significantly better value for money for me.
Seriously, be mercenary in your choosing, (and not just with mobile provider), and forget loyalty, grab the best deal you can from whoever you can.