I think that maybe because she had never been in a restaurant before,so maybe she can feel ashmed or something like that.In life,sometimes there are some people who had never done,gone something and when other people talk,invite,want..them to do,go..that,they can feel they can't,or they ashmed..So,i think that with Queen Elizabeth,it is normal.It is human feeling.
"God save our Gracious Queen,
Long live our Noble Queen,
God save The Queen"
God save Her Majesty Queen Elizabeth II