May not be the data point you're looking for since it didn't involve an LDR or US/UK relationship, but...
- previously engaged several years ago after 2 years of knowing each other and 18 months of dating. Broke up 10 months later (thank Goodness)
- currently dating an Englishman (in person), have been for about 8 months. Figure there's no chance of a proposal until post-November when he's met more of my family, and would be surprised if a proposal showed up before next summer at the earliest. We are living together, though, which is going quite well (so far).