By Aziz El Yaakoubi, Maha El Dahan and Yousef Saba RIYADH (Reuters) – The United States and Britain are ramping up pressure on Saudi Arabia to pump more oil and join efforts to isolate Russia, while Riyadh has shown little readiness to respond and has revived a threat to ditch dollars in its oil sales to China. British Prime Minister Boris Johnson flew into the world’s biggest crude oil exporter on Wednesday, a day after U.S. security advisor Brett McGurk arrived with a U.S. delegation. Saudi Arabia and its neighbour the United Arab Emirates, which are among just a handful of producers with sp…