From the Telegraph:
In 1222 the Council of Oxford declared April 23rd to be St George’s Day. However, it wans’t [sic] until 1348 that St George became the Patron Saint of England.
St George’s cross emblem was adopted by Richard The Lion Heart and brought to England in the 12th century. The king’s soldiers wore it on their uniforms during battle to avoid confusion.
From a more respectable academic work on the topic:
Olivier de Laborderie is quite correct in showing that Richard had nothing to do with the famous St. George’s cross. When the English King Henry II and the French King Philip Augustus met at Gisors in 1188 to plan the crusade, they decided that the English were to wear white crosses and the French to wear red ones. Although it might be tempting to think that Richard was still declaring St. George the patron saint of the English crusaders (since, as we have seen, George appeared to the crusaders at Jerusalem wearing a “snow-white” cross), it is far more likely that the crosses of the participants in the third crusade, which are identified only by their colors, were simply an easy (and arbitrary) means of telling who belonged to which army.
[Note: Knowing that the red cross later became the English national emblem, some historians have assumed that the council at Gisors must have assigned red crosses to the English and white ones to the French, but other contemporary sources, such as Roger of Hoveden and Ralph de Diceto, agree with Roger of Wendover about the colors of the crosses. John Rous (1411-91), the Warwickshire antiquary, recognized the significance of the decision at Gisors: “From this it may be conjectured that the English did not then hold St. George as their patron, or at least they did not then carry the arms corresponding to him.”]
In a similar way, the Council of Oxford in 1222 probably did not elevate the feast of St. George to being a minor holiday, thereby acknowledging George’s patronage of England, contrary to almost everything written on the cult in England. This point has been questioned by a number of people, including C.R. Cheney. There is a list of feast days allegedly promoted by the Council, but Cheney notes that of the seventy manuscripts of the canons published by the Council, not one contains the list of feast days. The list itself, moreover, commemorates St. Edmund Rich, who was not canonized until 1247, so it is probably spurious. Indeed, the first certain piece of legislation prescribing festa ferianda for the entire province of Canterbury, in 1362, does not mention St. George at all.