It looks like an assault that succeeded against a large body of (possibly) weak morale troops...added to the leader's differential that may explain things...but anyway hard to tell without the combat script and logs
I was defendind against a sortie. I captured a supplies wagon and 2 canons units. I had no chance of winning the siege before, but with those guns... How can I give you the combat script?
Go to your game's folder and send me the content of the Scripts (all those txt files named "Reports") and your Log folders. Plus the game save (of course of the turn when this occurred). See all those icons below the battle loss (and the number of Irregular troops you have in a forrested terrain), it looks like you ambushed the sortie (enemy superiority is worthless in forrest terrain with low frontage vs irregulars), which would explain the loss suffered by the brits...think "Monongahela 1755" and you'll have the explanation