The Princess MP could probably cover the OR 357 (am I remember number correctly?) except speed and flight height. Perhaps a derivative of jet Saro Duchess with RR Avon in place of DH Ghost could be a perfect match.
Not really, the RAF didn't really like flying boats in the post-war era. There were attempts to persist with them with support from some factions within the Air Staff, like the Saro P.104, but ultimately Coastal Command wanted land-based types.
Also, by the time OR.357 was formed the Princess and Duchess represented old 1940s technology. The Saro P.104 was actually quite an advanced design with its hydrodynamics and would have probably been a decent type but it was not to be.
I recommend Chris Gibson's Nimrod's Genesis
for the full story of post-war anti-submarine aircraft.