I can’t remember, are you still nursing your littlest? I had a friend whose older child had cancer, and the hospital were very supportive of her giving him expressed milk. If you’re not, you could try Human Milk 4 Human Babies. I’d be happy to express for you, but I have no idea how I would get it to you. Can look into it if it would be helpful though.
ETA: have had a quick look and there is a uk based refridgerated courier service. No idea of price, but if it’s something you need I’m more than happy to pump for you.