To be fair I'm not sure about 12/18 month deployments, I mean aren't there news reports about issues with the RN hulls having moved to 9 month deployments (among other issues), and even the USN has paid a price for extended tours in terms of support and maintenance issues.
That being said I totally agree about what the EPV/MRV should be used for if we go down that route rather than stay in EEZ operations.