Airlines hammer their aircraft, target utilisation is often 13 to 16 hours per DAY for a long-hauler. An RAF Sentry would barely achieve that in a week.
Also consider that the NATO E-3 force entered service in 1982 and will continue until 2035 ( though two have been retired ). They have invested in airframe and avionics sustainment. The Saudi E-3s have just passed 30 years in service and have no retirement in sight.
The MoD / RAF is just really, really poor at airframe sustainment. Excuses about the USAF having more money or more spares are just that, excuses; they 5,000 aircraft to maintain, overall, yet still manage to budget for long-life sustainment for types like the E-3.