Well, from White Labs website:
WLP023 Burton Ale Yeast
From the famous brewing town of Burton upon Trent, England, this yeast is packed with character. It provides delicious subtle fruity flavors like apple, clover honey and pear. Great for all English styles, IPA's, bitters, and pales. Excellent in porters and stouts.
Optimum Fermentation Temperature: 68-73°F
Alcohol Tolerance: Medium
If it's been at 55 the whole time, you're lucky it fermented as much as it did! I'd bring it up to the high 60's and let it finish up, though! Depending on what the gravity was when you started, it might be nearing completion.