By Michael Holden and Kate Holton LONDON (Reuters) -A beaming Queen Elizabeth waved to cheering crowds massed outside Buckingham Palace on Thursday as Britain kicked off four days of pomp, parties and parades to celebrate her record-breaking 70 years on the British throne. The 96-year-old monarch appeared on the balcony alongside her family, but the palace later said she had experienced a recurrence of mobility problems and some discomfort, and would no longer attend a Service of Thanksgiving on Friday. The queen has reduced her public appearances in recent months due to what Buckingham Palace…