UK Zone Map
The RHS is using a different Hardiness rating system, which takes into account of greenhouse / conservatory conditions.
All ratings refer to the UK growing conditions unless otherwise stated.
H1a - Under glass all year (>15 deg C)
H1b - Can be grown outside in the summer (10 - 15 deg C)
H1c - Can be grown outside in the summer (5 - 10 deg C)
H2 - Tolerant of low temperatures, but not surviving being frozen (1 to 5 deg C)
H3 - Hardy in coastal and relatively mild parts of the UK (-5 to 1 deg C)
H4 - Hardy through most of the UK (-10 to -5 deg C)
H5 - Hardy in most places throughout the UK even in severe winters (-15 to -10 deg C)
H6 - Hardy in all of UK and northern Europe (-20 to -15 deg C)
H7 - Hardy in the severest European continental climates (< -20 deg C)