I hear ya!
I know to the millimeter what I like in a frame fit. Just looking at some good pictures and the geometry chart I know where I'm going to be on it give or take a smidge (seat tube angle as the main variable and doing trig it's easy to calculate). I can usually spec my seatpost setback, stem length and bar throw & drop and be right on home plate or so close only minor tweaks are required.
The big unknowns for me are handling, steering and ride. The Ribble, for example is quick on turn in! Really quick. It rides very much like my Wilier's yet stiffer in the BB. It's less chattery than the Emonda. Another surprise was the slightly longer WB eliminate the toe clip overlap of both the Wilier's and the Emonda.
I also had the benefit of a few reviews. I relied on reviews of Ribble, the company, more than the bike reviews.
Most outfits have a great return policy, but back to England? No way. You are quite correct to be conservative in purchasing sight unseen! That test ride might cost you $100 shipping to you and $100 shipping back across the pond. No way.
