My wife was in the same situation, trying to renew her British passport when she was told the rules regarding surnames had changed.
She decided to wait until the covid situation in Thailand got easier before going back home to sort it out. Meanwhile she wrote a letter to the UK passport office saying it was difficult to change her surname at this moment in time either by going to Thailand or by doing it through power of attorney.
The passport office agreed and sent her a new UK passport. Maybe they're softening the rules at the moment due to the pandemic.
