If you don't want to see your GP, why don't you go and see a podiatrist for an initial consultation? No need to commit to anything. They may recommend you see your GP for further treatment.
If it were me, I would visit my GP first. I already wear orthotics in my shoes, which were recommended by my physio and provided by the NHS. They aren't even visible, but give more support and their purpose is to realign my foot.
They were a bit uncomfortable at first, but you have to persevere. I am now on my third set.
Please don't leave it to get any worse, it will be even more difficult to fix.