Pots you say
Do the pots have holes on the bottom ??
Without
good drainage all that English rain and water cause root rot.
Before the plants start dying, to they yellow their older, lower leaves and the yellowing works upwards
And check the pH levels. Fertilizer may make the soil acidic for a base loving plant and vice versa. Every apothecary should have pH tests and then just google what pH the plant thrives in and compare your findngs.