Helth Cover

My mom is visiting me for 4 weeks from SA. I have been trying to get her medical insurance but they wont pay out much for any pre-exisitng conditions. She has high blood pressure and angina. If she becomes ill while visiting me here in the UK, will she be covered by the NHS? Can anyone give me advice for this??
I completely agree…why should a foreigner benefit from free health care? But this is not our intention. We just need assurance that in the event of an emergency, the NHS won’t turn her away if she does not have cover. We were told by the travel agent that we are wasting our money taking out medical insurance for her because of her condition. (ie, insurance company cover very little for her condition) Do you see my dilemma? Do I take my chances and pray damn hard nothing happens to her while she is here? or take out the insurance knowing that the chances are we won’t be fully covered anyhow? What will the financial implications be if she has to use the NHS? She is only here for 4 weeks and we are not trying to exploit anybody.