Dickwella smashed 95 and Chandimal 80 to help Sri Lanka register their highest-ever ODI total against England and the tourists slumped to 132-9 when rain stopped play in the 27th over.
It was only a consolation victory for the hosts as England, who suffered their biggest ODI defeat, took the weather-plagued five-match series 3-1.
England s previous heaviest loss came against the West Indies by 165 runs in Kingstown in 1994.
England captain Eoin Morgan, who received the man-of-the-series award for accumulating 195 runs in four games, rested himself for the final match, with opening bowlers Chris Woakes and Olly Stone also given the day off, while Jonny Bairstow was still missing after suffering an injury playing football.
“Sri Lanka took advantage of a very poor day from our part. Very disappointed to end the series like this,” Morgan said after the loss.
“Individual form aside, what was best for the team was that younger players got some games under their belt. That was progress for the squad before the World Cup.”
Akila Dananjaya took 4-19 with his mix of leg-spin and off-breaks, while fast bowler Dushmantha Chameera claimed three wickets to flatten England s chase.
Chameera removed three top-order batsmen — both Alex Hales and stand-in-skipper Jos Buttler for nought and Test captain Joe Root for 10 as England slipped to 4-3 and 38-4.
Ben Stokes and Moeen Ali, who made 37, put up some resistance during a 79-run partnership for the fifth wicket but Dananjaya sent the pair back to the pavilion.
Stokes also seemed to have injured his calf during his 60-ball 67 as the all-rounder needed regular physio attention and limped to his half-century.
Dananjaya finally ended his agony as Stokes holed out to long-on and the rest of the batting fell like a pack of cards.
While Sri Lanka took their catches and were brilliant with their ground fielding, the visitors were sloppy during the home team s innings.
Tom Curran put down Chandimal off Ali when he had made only six, and it proved the costliest miss.
But it was the left-handed Dickwella who gave the hosts a blazing start in a 137-run opening stand with Sadeera Samarawickrama, who made 54.
Chandimal soon took charge to continue the momentum, building crucial partnerships including a 101-run third-wicket stand with Kusal Mendis, who smashed 56 off 33 balls.
Paceman Tom Curran removed the captain with a slower delivery that Chandimal could only hit to Jason Roy at deep mid-wicket.
Curran struck again on the very next ball with the wicket of Thisara Perera for 11, but was denied a hat-trick by Dananjaya, who played a useful cameo of 18 not out.
Dananjaya put on an unbeaten 38-run partnership with Dhananjaya de Silva, who hit 19 off 15 balls.
“The dead rubber game is never easy to play but we did really well,” said Chandimal.
“We are a young team and we will keep improving in every game. We are looking to find consistency. We are looking forward to the next series.”
The two sides will next play a Twenty20 international on Saturday before a three-Test series.