Harry didn't have to walk. Prince Philip said he would walk with him if Harry walks. Worried Harry may regret it later in life if he didn't walk.The other thing I wanted Tom to ask was did Harry and William ever say they didn’t want to walk behind the coffin. He says they were told that was the decision
If they told Harry he couldn't walk, he would have complained bitterly that they allowed William, the heir and not Harry, only the spare