By Sachin Ravikumar and Farouq Suleiman LONDON (Reuters) -Tens of thousands of British nurses will go on strike for the first time over demands for better pay, their trade union said on Wednesday, adding to pressure on Prime Minister Rishi Sunak during an economic crisis. Nurses at the majority of state-run National Health Service (NHS) employers across Britain have voted to strike, the Royal College of Nursing (RCN) said in action that threatens major disruption to an already strained health system. The RCN, which has more than 300,000 members, said industrial action would begin before the en…