Travel insurance for stage 4 melanoma skin cancer

My husband has stage 4 melanoma skin cancer, he has been undergoing immunotherapy for 3 mths which has not worked. He is now taking cancer inhibitor drugs. Has anyone managed to get travel insurance with stage 4 cancer? I’ve done a bit of research and it seems impossible. ( when it was stage 3 it was £2000, but now I can’t even get it) Does that mean we are no longer able to travel?