Temperatures may have hit a record in July but a chilly August made this summer the coldest in the past three years.
Higher rainfall than average also ensured a soggier season than usual, finishing up with a wet bank holiday weekend at the end of last month.
The average temperature for summer was just 13.9C (57F) - a far cry from the sweltering heat of July when the mercury rose to 36.7C (98F) at Heathrow.
Low pressure across the UK meant August was generally unsettled and cool with eastern England enjoying the best if any warm weather.
It was the wettest end to the summer in 50 years for Dorset, Hampshire and Guernsey, the Met Office said.
Cornwall and the south coast had to cope with up to three times the average rainfall for August.
Chief Scientist Professor Dame Julia Slingo said: "No-one can deny that we have had a pretty disappointing summer with a lot of unsettled weather and only a few warm spells, especially through July and August.
"Our weather has been dominated by low pressure over and to the west of the country that has brought us periods of heavy rain from the south - what we call the Spanish Plume."
She said early predictions that temperatures and rainfall would be near normal were proved wrong.
"As the season progressed all the leading models around the world failed to capture the signal for unsettled weather over the UK.
"We all know that forecasting months and seasons ahead is still in its infancy and much more research needs to be done."
It is hoped the use of a new £97 million supercomputer, the first phase of which went live this week, will allow more accurate weather forecasts in future, she added.