A quick (about 2 hours) AU British multi role fixed wing aircraft (ASW/AS/COD/Tanker/AEW) started from P.139 Guardian. It is equivalent to US S-3 Viking. However, RN resctrictions was to be capable of operating from small carriers (Audacious and Hermes). This mean that landing speed must be as low as possible (solution was boundary control, like Buccaneer), maximum length 54ft, folding wings and strongly built to attaint violent speed decrease. Capapults of these carriers could handle Buccaneer, so weight can be up to 24-25tn. Engines were derived from RB.162 core but with a fan in front of (something like a scale down Tay). I estimate length about 53ft and 71.5ft wingspan (folding to half in Gannet way) and 24-25tn loaded. Four variants built and entered service gradually. First was the COD, with a ramp like door at the back, followed by tanker (which featured the same system with Buccaneer) and ASW/AS, using the electronics from modernised Gannet. Last variant was the AEW, again using the radar or the Gannet AEW (variant AEW.7). Later, modernised ASW/AS platform was capable of taking two Sea Eagle missiles, while it has retractable MAD and sonoboys bay similar with US S-3 Viking. Torpedoes are carried as S-3 again.
Another possible platform could use a scale down fuselage from WG.22 with different windscreen and S-3 Viking wings (a interesting mod which I would try). An interesting thing is that there was off the shelf Rolls Royce engine equivalent with TF34, the RB.163 Spey Junior as used from Fokker F.28-tweaked of course for sea use.