Hello
you should be able to purchase one from the shop. But before jumping into that - do you have a MyFairphone App installed on your device, could you run some troubleshooting?
I just have had a quick look on the relevant troubleshooting process via self-repair portal and they also suggest running internal tests - so that could be one of the first steps
Finally, you mentioned the UK, you may try contacting the local Angels, perhaps they could try charging your battery and/or device; or ask at local communities topics