By Sam Hall & Bradley Jolly Royal fans should expect an “inevitable reduction” in official engagements with both King Charles and Princess of Wales battling cancer, an expert said. Princess Kate joined the monarch in sharing her cancer diagnosis on Friday and, in a poignant video message, the mum of three said she is now undergoing preventative chemotherapy. As a result, she and Charles, 75, are likely to be out of action for some time, a blow managing editor of Majesty magazine Joe Little said will be a “significant moment for the monarchy”. He added Kate’s bombshell announcement was a “great…