I worry that your correspondent doesn't grasp the distances involved, but how about:
Places always worth a visit: RHS Wisley,RBG Kew, Bedgebury Pinetum.
Seasonal Interest: a top time for Cornish Magnolias: Lanhydrock, Trewithen, Caerhays.
Best Landscape Gardens: Stourhead, Studley Royal, Stowe.
Best Formal Gardens: Levens Hall.
English Classics: Sissinghurst, Hidcote, Dixter, Kiftsgate.
Crazy Scottish: Drummond Castle, Pitmedden, Edzell.
......how many years does the person have?