Kind Kim Frankenhoff came to the rescue, having been handed a saucer, and she came up with the following:
The set is (as identified on an assortment of dish bottoms) Royal Vitreous made by John Maddock & Sons. And (yay, Kim!) the pattern is officially known as Mad6. The startling information comes from an eBay search, where I find the Oval Covered Vegetable dish (above), for example, is priced at $245. And the Round Footed Soup Tureen is $375. On the other hand, the Round Covered Vegetable Dish is a mere $195. Ah, and a cup and saucer set (a couple above; I have 8) is $32 @.
This does not bode well for the goal of filling out the impulsively purchased set with a gravy boat. Mind you, people offering random pieces on eBay are probably not those who can even identify the pattern, so there'd be a posting something along the lines of "Old Gravy Boat RARE!" and I'm not about to spend the time. The prices in the preceding paragraph are those from online stores that specialize in identifying and selling this sort of thing, and I'm not about to spend the money for their expertise. So probably what I have is what I'll stick with, though I'm still curious as to when it was manufactured and what was in the "complete" set. (Old china can be weird; Valerie gave me a bunch of Haviland china a few years ago in what a friend identified as the "Apple Blossom" pattern -- and it included ramekins and strange dishes for fish bones. Heck, a cup and saucer set from that pattern is listed at $60 from Replacements Ltd.)
I love the idea of even having a Round Covered Vegetable Dish, and it's been used enough that I trust my dish is not truly worth the full $195. But this is a hobby in which I've probably dabbled to my limit already.