The answer is no it is not a requirement for a UK fiancee visa to buy rings to prove you are engaged they are more interested in plenty of sustaining proof about your relationship to show that you are an ongoing relationship and not manufactured and you do this with photographs, e-mails, telephone records etc, it is nice to  buy engagement rings to show commitment and looks nice in a UK fiancee visa application when you take a photo but it is not a requirement, would I do it the answer is yes.