If these guys get convicted and punished in the US, even if let off and told to leave the country, I assume the British Army will then charge them
with various offences ?
If by 'charge' you mean prosecute them, then no. They may, however, face administrative action, which is entirely separate from the military criminal justice system. Administrative action may result in a range of outcomes from an informal interview or rebuke to, in the most serious cases, termination of service.
A change in the terms of the Rehabilitation of Offenders Act 1974 means the Army can no longer take administrative action against soldiers who receive police cautions. If, therefore, the soldiers were to receive in the US something akin to a UK police caution (very unlikely, given the seriousness of the alleged offences), administrative action may not be an option.