The scribal abbreviations include both abbreviations for whole words and abbreviations for commonly-repeated sequences or case endings. So once I started immersing myself in that world, I began slowly to expand the inventory of signs in the system. Once you're already thinking of words like "&" it's not a huge stretch to stick a new symbol in for, say, "is"—I picked one of the abbreviationes for "esse", I think. Then "we", "they", and so on down the line.
And I had already been thinking about sequences like ff and ee, so it was natural enough to expand further into common letter sequences in English. English doesn't inflect nearly as much as Latin, of course, so the series of relatively fixed noun and verb endings wasn't going to supply itself—nevertheless, it turns out that English spelling is so idiosyncratic and crufty that there's quite a bit that one can compress!
th Is maybe the best and most obvious example; it has been represented already as a single letter in earlier forms of English, a couple times as it happens. So it's trivial to choose one of those letters and bring it back into the present. Make it ð, which is easier to write than its rough synonym Þ. After that you might as well bring back the yogh, ȝ, for gh.