LONDON (Reuters) -- Britain faces its largest ever strike by health workers on Monday as tens of thousands of nurses and ambulance workers walk out in an escalating pay dispute with the government, spelling further disruption for an already strained health system. Nurses and ambulance workers have been striking separately on and off since late last year but Monday’s walkout involving both, largely in England, will represent the biggest in the 75-year history of the National Health Service (NHS).