The flight of Warwick and Clarence to France took place after their complicity in the rebellion that culminated in the Battle of Empingham / Losecote Field. With their intent having been discovered by the king and the army raised by Sir Robert Welles defeated and dispersed, the senior nobles fled. First, they travelled to the North West of England. There they hoped to attract support from Thomas, lord Stanley. However he declined to join them against King Edward. With that they took to the sea, eventually gaining refuge in France.
Other relevant pages: Edward IV, Flight of Clarence and Warwick, Clarence becomes disillusioned, Battle of Empingham/ Losecote Field, Contemporary view on the readeption, Clarence, Gloucester and the marriage to Anne, Attainder of George Duke of Clarence, Trial and execution of George duke of Clarence.
The Flight of Warwick and Clarence
After the departure of the nobles before-mentioned from London, the men of the county and district of Lincoln, for the first time allying themselves, as it were, with the Kentish rebels, and resisting the laws and customs of the country, appeared in arms, under the command and guidance of the son and heir of the lord Wells. King Edward, however, having levied an army, as soon as he had arrived at Stamford, at the same instant, both saw and conquered them. All the leaders of the hostile force fell into his hands; and after inflicting capital punishment on them for their misdeeds, he showed grace and favour to the ignorant and guiltless multitude. Upon the news of his having gained this great victory reaching the ears of the duke and earl the noblemen already mentioned, being fully conscious of their share in promoting this insurrection, they consulted their safety in flight; upon which, the king followed in pursuit of them, along their route from the county of Lancaster across the intervening counties, until they had arrived at the city of Exeter in the county of Devon. Having arrived here before the king could come up with them, and finding a few ships in [unknown], they embarked; and after spoiling of their property, in ships and wares, all the Hollanders and other subjects of the duke of Burgundy they could meet with engaged in mercantile pursuits, they pushed on with the utmost speedy and at length, with their confederates, landed safely in Normandy.
A coloured manuscript map of the coasts of Devon and Dorset from Dartmouth to Weymouth: Royal MS 18 D III, f. 10r