PN, if only it was that simple to build new warships.
Unfortunately, as an example it would be virtually impossible to build new Type 23s as most of the Jigs have gone & many of the equipment suppliers are out of business. A new "Type 23" would probably need re-drawing & fitting with different machinary & sensors; therefore, like the Nimrod MRA4 looks a bit like an MR2, it might look like a T23 but would be a different beast really.
wrt Sandown & Hunt class MCMVs (mine hunters) then they are impossible to replace as the moulds for the GRP hulls were binned a long time ago. etc.