LONDON, Feb 5 (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. Asked by Sky News if the strikes would put lives at risk, business minister Grant Shapps said he was "concerned that it does" because of a lack of cooperation between the back-up services, such as the army, and those workers who are striking. In Wales, nurses and some ambulance workers have called off strikes planned for Monday as they review pay offers from the Welsh government