Exactly why would we need a UK designed, built, and crewed Ship? - it would be very very cheap to acquire
Who would maintain, service, and repair the ship? - an excellent point - neither country has any spare capacity TBH and the constructors probably binned the blueprints (or sold them to the Chinese) already
Why would the RN accept being seconded to the US Navy? -a) because the Govt tell them to b) its the only way they'll get any experience as the cash has run out in Whitehall