Mixed sex friendships

I will post more on this subject at another time, but there's one thing I had to mention with respect to myself and E. A few days ago, we were in a town across the country where we met this really lovely girl (both physically and personally... if only I'd had an extra day with her, I think there could have been something, but typically our time was limited). She was really interested in our relationship as platonic friends as this was a concept she found fairly alien (we're British, she's Australian if that make any difference).

E and I engaged in some banter, which our new friend found really funny. She made a comment about our relationship which prompted a reply from myself and E. E mentioned that she was essentially a little sister of mine whereas my reply was that we were pretty much a couple in everything but sex (and to be honest, why was E telling this girl what my relationship was with her, surely she should have been saying that she regarded me as a big brother figure which is her own opinion?) My reply wasn't meant as a vindictive reply, quite the opposite, in fact as I believe the relationship I describe is actually closer than the brother/sister one E describes, but I think our new chum did agree with me based on the couple of hours of interaction she saw between myself and E. I think that might help pinpoint the differences in viewpoints of our friendship (see the comment above about why E didn't voice her opinion but tried to influence mine). This girl wasn't the first to have made comments - prior to E's emigration, a fair few people had commented on us, wondering if we were a couple.

I've been, at best, confused about our friendship for the past few years, often wavering about whether my feelings crossed the "platonic" line or not (and I've been concerned about hers until the recent conversation, but things have become clearer since then). At the time, I thought they did, but I'm not quite sure right now. But, saying that, I'm not entirely sure my friendship with E is 100% platonic, at least from my point of view. 

From what I can tell, E is 100% certain that she's in a platonic friendship and that things will never change and we will remain like this forever. However, my opinion is that E ticks the majority of boxes of a girl that I'd be attracted to and while I might not be romantically (or sexually) attracted to her now, that could change tomorrow, or next week, or next month or... well, you get the picture.

Ignoring any possible non-platonic attraction, there is a level of, I dunno, comfort in the friendship that I've not really had before and can't see ever having again.

She does seem to get very defensive when it's mentioned that we could end up as a couple/fuck buddies... when I told her that SSCW had asked if we were going to get together on my trip, she was quite agitated, though questions from our friend mentioned above are treated amicably, though she will go to lengths to describe how we're not dating. Is it a case of the lady doth protest too much?