Re: Turkey is a NATO country.
I don't think there's anything to stop the UK doing all its overhaul work itself, except for the substantial investment involved.
...or that it might be a breech of the terms of the purchase contract.
Sure, the UK might have the technical ability to perform overhauls, but they might be missing the legal - contractual, patent licensing and copyright licenses - to do so. Not to mention possible software keys to also do so. For example, replacing a component on the engine might be detected by the engine firmware or aircraft flight/control systems, and might refuse to function until the appropriate license key is entered to show that it is an approved repair - a key only supplied to approved repairers like the Turkish.