I am a pommie, so lived in Buckinghamshire 35 years.
Jimmy's recommendation:
West country in September. We always had 3 weeks holiday in Sept in Cornwall. LOVE those wild blackberries.

July in Scotland still light at 10 pm We went there - 1995 and it was a heatwave

May in southern England - beaut, all the blossoms are coming out, but there can be frosts up till end of month.

November, December, Jan, Feb - forget. Cold, dreary, fog, look now on the news broadcasts. Gets dark at 3pm.
March? As they say... "In like a lamb, out like a lion. In like a lion, out like a lamb." Daffodils and snowdrops pretty though.
AVOID LONDON AT ALL TIMES. NO longer the England I remember.