Rule 5(1) states that the buyer (or seller as the case may be) must assent to the unconditional appropriation.
However, Rule 5(1) also states that the assent may be implied and may be given before appropriation.
In Healy v Howlett & Sons the court seems to have proceeded on the basis that the buyers assent would have been implied if the goods had been ascertained.
Other examples of implied assent - see
Pignataro v Gilroy (1919)
Aldridge v Johnson (1857)
It may appear from the cases that in practice all that is needed is an appropriation of the goods and that it need not be unconditional this is not always the case - see
Carlos Federspiel & Co SA v Charles Twigg & Co Ltd (1957)
Cf Hendy Lennox (Industrial Engines) Ltd v Grahame Puttick Ltd (1984)
Remember ss.16 and 17 have priority over s.18 Rule 5 so the courts may imply intention under ss.16 and 17 rather than rely on s.18 Rule 5 - see
Wait & James v Midland Bank (1926)
Stapylton Fletcher Ltd, Re (1995)