majorincident refuse to see that the problem is specific and not the whole of the internet
Unfortunately, the problem is very specific, to you and anyone else that's chosen to continue using outdated hardware or software.
You are in a very small minority and you can't really expect any developer to spend resources trying to fix a problem inherently caused by using an old non-compliant browser. The time and effort required to make things work for you, and the other tiny minority, would be better spent improving things for the majority.
This isn't personal, and I'm sorry if you end up taking it that way, it's certainly not my intention to upset or annoy you further, I'm simply explaining how this kind of project management must work.
You have only 2 choices with regards the giffgaff forum, either learn to use it as is, with your aging hardware and software, learning how to overcome or ignore the incompatibilities, or gain access to a device which will work fully.
Any mobile device, such as a cheap Android phone or tablet of less than 3 or so years old ought to render this site fine, alternatively Chromebooks are relatively inexpensive, or even a cheap 2nd hand laptop running Windows 7 or later ...
If you decide not to upgrade, you must expect more and more websites to become unusable, or unstable, over time as web technology moves forward.
I hope you find a solution that works for you.