Unless you get a sensible response from Nexgen, I suggest you start the Small Claims process against mGage Ltd. Tell them that you do not accept that any interaction between your phone and their service represents consent as it was probably the result of exploits on a web page or malware.
I tried calling 02037403517 and got a message to call back between 9:00am and 5:00pm Monday to Friday. It is not permissable under the rules to get you to call a premium rate number OR ANY NUMBER OUTSIDE THE UK.
mGage took the money on behalf of Nexgen, and they don't dispute that fact, so you don't need to prove negligence on their part. It IS worth scanning the forums of the networks and Twitter to find other cases similar to yours with Nexgen as the "service". You will find there are quite a few. Use these to point to the fact that yours is not an isolated case.
A sample letter before action can be found here: https://payforitsucks.co.uk/im-not-getting-anywhere-with-my-case/#letter
Let me know if I can help you further with this process.
As I have said, I completely agree with you that the attitude of GiffGaff is morally reprehensible and legally very dubious. They are aware that they are handing members money over to known scammers, and I believe that their failure to act to protect members amounts to negligence.
It is, of course, open to you to pursue GiffGaff in this matter. To do this you would have to show negligence. There are a significant number of complaints on this forum about Nexgen, and GiffGaff can hardly claim to be ignorant of the fact that there is a problem with this "service". It would be for a court to decide whether continuing to allow this company free access to members accounts amounts to negligence. I think it might.
There is nothing to stop you from starting a Small Claim with GiffGaff as defendant. If you choose this course, the payforitsucks website will do its best to support you. I think there is a very good chance that you would win.
However, in honesty, your claim against mGage is easier to provel as you have no need to allege and prove negligence.
Got some more info on the fictional 'service'... http://m.uk.mycontent.media/#/personalisation1 ***DO NOT OPEN THIS ON A MOBILE*** it is the page which if opened subscribes you.
MyContent Age 16 or over.
Send stop to 64055 to unsubscribe at any time.
MyContent is a subscription service. Charges will be made to your mobile bill or deducted from your prepaid account. Carriage fees or data charges may apply. Contact us; contact<at>nexgenmobi.com. or 0203 740 3517. By Clicking Subscribe you agree to be subscribed for £4.50 per week and that you are the owner of the device and are at least 16 years old or have the bill payers’ permission and agree to the terms and conditions. You will get unlimited access to the newest content. To opt out of the service sent STOP to 64055. This service is bough to you by Nexgen International 5827 Corner of Graduate Crescent and Bachelor Avenue, Belize City, Belize, Central America. Reg: 163347
There is no 'product' and you do not receive any access to 'content'.
Have you managed to resolve this?
Draw their attention to the Payforit rules which allow for disputes to be escalated to your network.
You also have the option of using the Small Claims procedure to get your money back from mGage Ltd.
Keep me posted of your progress. Happy to help if I can.