I went back to episode 8 to rewatch the specific scenes. Here is what happened:
1. Penelope enters Marina's room, to find Marina packing, convinced that she is no longer pregnant. Marina says that she should have felt something from the pregnancy by now, suggesting that at this point, she's probably about four to five months along.
2. They can hear a horse/carriage clattering outside. Penelope says, "That is odd; we weren't expecting any callers today," and goes to the window to look down. Marina joins her. They look down.
3. Sir Philip steps out of his carriage. Penelope and Marina can clearly see his face and those of the footmen and the carriage driver.
4. When Marina sees his face, she makes a small sound.
5. Penelope says, "Marina, are you quite well?"
6. Marina backs away from the window.
7. The scene cuts to Daphne's boring life.
8. Lady B and Daphne run into Lady F. Their conversation is interrupted when the servants say that a Mr. Crane - not a Sir Crane - has arrived at the house. Daphne assumes this must be George.
9. Daphne, Lady F. and Lady F's maid return to the house. (Lady B is not in this scene, for whatever reason.) Lady F says:
"Mr. Crane, how delightful to make your acquaintance. I had no idea a soldier such as yourself was coming to town. And, well, Miss Thompson has not received any correspondence from you in quite some time."
Sir Philip: "Nor would I have expected her to, my lady."
Lady F: "I beg your pardon?"
Marina: "That man is not who you believe him to be, Lady Featherington."
Philip starts explaining things; Marina flees the room.
10. Marina says that a) Philip found a half-written letter addressed to her, but, b) could only find out where Marina was because of Daphne's enquiries.
11. Marina then tries to convince us that George wasn't a complete jerk.
12. Cut to Eloise/the modiste and the boxing people. When we return to the Fs, Sir Philip wants to speak to Miss Thompson.
13. Scene, including two footmen, Sir Philip, Marina, Lady F, Penelope, and the other two F girls:
Marina: Marry you. I do not understand.
Philip: After my brother clearly took liberties with your [pause] virtue, I would merely be doing my duty. [Marina looks horrified.]
Lady F: A respectable marriage! Miss Thompson! What a fine thing! Sir Philip inherited the Crane title. He has a perfectly adequate estate to support you and perhaps your distant cousins too --
Marina: I cannot. I do not know this man. I do not love this man. I cannot marry this man.
Philip: George cared for you very much, Miss Thompson, and that will never change. But he would have wanted you to be looked after. He would have wanted you to be supported. Allow me to realize my late brother's wishes.
Marina: I thank you for your offer, Sir Philip, but I believe you should now take your leave.
Philip: Very well. I wish you all the best, Miss Thompson. [Philip leaves.]
Cut to Daphne/Simon, a family scene with the Bs, and the boxing match.
When we return to the Fs, we see them spending some of the boxing money on new clothes; Lady F says that Philippa has a dowry again. A couple minutes later, Marina realizes that she is still pregnant. Preparations continue for the Hastings ball. Marina confirms that she's still pregnant. She does not go with the Fs to the ball. The Fs return to find the servants and Bow Street Runners in their downstairs hall; Lady F asks about Miss Thompson, who appears quietly on the stairs, and learns that her husband is dead and the money is gone. Cut back to Simon/Daphne, Colin taking off, and Penelope and Eloise, before returning to Marina and Lady F.
Marina: How did you do it? How did you endure 2 and 20 years of marriage without love?
Lady F: You find things to love, my dear. Small things. Big things too, like your babies. And eventually they add up to be enough. You are strong, Miss Thompson. Perhaps even more so than me. You will do well.
Marina rides off in a carriage with Sir Philip, without saying anything else.
That's exactly what happened, word for word.
We can perhaps infer that Marina does recognize Sir Philip when he steps out of the carriage. But we don't know that - especially since there's at least three other reasons for her to look concerned and step back:
1. She recognizes Sir Philip's servants (again, they are visible from the window, and may have previously worked for George). They aren't with George. Bad sign.
2. She realizes that the man that just got out of the carriage has a strong resemblance to George - but isn't George. Bad sign.
3. She has a sudden bad feeling about this (which, given that the owner of the house that she's currently staying in is about to get murdered, and that she knows the Fs are having some financial difficulties, is a reasonable response.)
Meanwhile, we have several indications that Marina and Sir Philip have never even met before:
1. Sir Philip's proposal, which is entirely about his brother, not Marina.
2. Sir Philip not once saying something along the lines like, "Well, yes, this is unexpected. But when we met back in X, I thought you were quite charming."
3. The servants not knowing which Sir or Mr. Crane this was. (That doesn't rule out a previous meeting, of course, but also suggests that Marina didn't enter the room and go, "Oh, Sir Philip!")
4. Marina telling Daphne - and us - that Sir Philip would not have found her if Daphne hadn't started to make enquiries. Sir Philip had George's letter, addressed to Marina - and yet had no idea how to find her. That more than strongly suggests that he had no idea who Marina was. "Marina" is not that common of a name.
5. Marina not writing Sir Philip to find out what happened to George (see the whole Jane Austen thing about unmarried women not writing to men, which would be especially true if the two of them had never met).
6. And the biggie - Marina flat out saying that she doesn't know Sir Philip, something Sir Philip doesn't contradict.
And again - Jane Austen gave us plenty of examples of people never meeting family members of people they know well in at least three books (and arguably Northanger Abbey). So, yeah, based on what was said and not said, I think it's at least possible - even probable - that Marina and Philip had never met before episode 8.