I'd be more than happy to evaluate a British or European design that could step in as a true Sentry replacement. The naked truth is precisely zero manufacturers would be be prepared to fund, develop, mature and prove such a capability. What they are happy to do is to spend MoDs money to fund/develop/mature their platform over an extended period whilst we continue to spend more MoD money sustaining the current platform.
The UK MoD just does not have the funds to support domestic industry and HM Treasury will not pay extra to do so.