Fag tells the Coachman that Jack "got acqainted with Miss [Lydia] while on a visit in Gloucestershire." But unless I have missed it, we do not hear who he was visiting. My guess is that Jack and Lydia were introduced through Julia (Lydia's cousin) and Faulkland (Jack's friend). The only alternative that does not rely on unlikely coincidence is the reverse, i.e., that Jack and Lydia introduced Julia and Faulkland. I cannot give a reason why, other than that I have the impression that it is the former, not the latter, because Julia and Faulkland seem a longer-standing relationship.

If Jack and Lydia were introduced by Julia and Faulkland, then it would perfectly explain how Jack knew ahead of time of Lydia's insistence on marrying a poor man. We know that Faulkland is in on Jack's impersonation when we first see Faulkland. Unless Julia is a total liar in Act I, scene ii, and I don't think she is, she has no idea about Jack's impersonation. So I think that Faulkland mentions Lydia to Jack, and then Faulkland and Jack scheme together to set up an "accidental" meeting of Lydia with Ensign Beverley. Perhaps Fag is in on the planning stage of this, we already know from Act I that Fag can sometimes play a significant role in dictating his master's choices.

I'd be very interested to hear what others think about this, perhaps there are clues i have missed which shed further light on this subject.

And in this regard, I do have a question, which I don't recall being answered earlier in this Groupread. Wouldn't an ensign wear a different uniform than a captain? If so, wouldn't Jack have had to be sure to always wear an ensign's uniform when seeing Lydia, but a captain's uniform when seeing his father? How does he manage this, is he a quick change artist? ;)

If it hadn't been for the Julia-Faulkland connection, my second guess for where to look for someone who might have brought Jack and Lydia together would have been Fag and/or Lucy. We know them to be clever, we know them to be familiar with each other by the time they exchange barbs in Act II, it makes me wonder whether they might have crossed paths of intrigue in the past. But I think that we are not meant to have to dig that deep, and assume that much, in order to answer your good question.