Some policies will cover you when you’re abroad if you have international cover included. PMI generally provides you with UK cover, so normally the best solution is to buy travel insurance that covers you for medically necessary healthcare while you’re overseas. You can then use your PMI once you are back in the UK.
If you are travelling in the EU or Switzerland, you can also apply for a UK Global Health Insurance Card (UK GHIC) or (if you have rights under the Withdrawal Agreement) a UK European Health Insurance Card (UK EHIC). If you have an existing EHIC from before the UK left the EU, it remains valid until the expiry date and works in the same way as a UK GHIC. Both cards cover you for things like emergency treatment, treatment for long-term or pre-existing conditions, routine medical or maternity care, oxygen therapy, or kidney dialysis. You can find out more about these cards here, but it’s always a good idea to supplement them with appropriate travel insurance cover.
If you require further treatment once back in the UK, you can either receive this through the NHS or use your private medical insurance to access private treatment.