Yes, Sybil wanted to “get away” from Downton, but do not mistake that desire into thinking that she didn’t love Branson. If you look back at the previous moments between these two, especially the night in the garage when she caressed his cheek, it’s obvious she has feelings for him, it’s written all across her face. What holds Sybil back from answering right away is this worry that the family she deeply loves will turn their back on her for the decisions she’s made for her life–decisions that include living a very different live (and a large part of that is marrying a man they wouldn’t approve of)–if you go back to Sybil’s conversation with Mary, where she tells her sister about Tom being in love with her, Mary’s reaction is to belittle Sybil (“Don’t be such a baby, this isn’t fairyland”). It wasn’t for a lack of love that she didn’t answer Tom right away…it was because she was scared of the reality of losing the love of her family and having to accept the reality that they would turn their back on her (or so she believed, at the time).
Sybil is still being “secretive” at this moment; she’s not telling anyone the whole truth. She’s coming to the decision that she will marry the man she loves (something which has been building and finally reaches the tipping point when she observes Mary and Matthew, who she knows love each other, but choosing to marry other people–she doesn’t want to follow in their footsteps, she doesn’t want to make that same mistake). But in order to keep people from stopping her, she doesn’t blatantly state, “I’m going to marry the chauffeur”, she simply says “I think I’ve found a way to escape”, which…is true.
She’s accepted the reality at this point: she can’t have it both ways, she can’t have the life she wants with the man she loves *and* have her family’s approval. And she chooses Tom.
Musings of a Yankee Countess 