Lady Thatcher is in pretty bad shape herself. She's had fainting spells, etc. I think that she has been asked by her physicians to avoid further public appearances.
I don't think she should attend. A long flight to the U.S. would be difficult for someone with her infirmities. Think of the problems that can arise such as blood clots in major veins, etc. I hope the Bushes have taken her under their wing and are putting her up at the White House and are transporting her in a comfortable, air-conditioned limo to the funeral service. What guts this lady has! I've admired her for years.