Thanks for all the replys!
The pattern does call out 8 sts. per inch. I just finished another vintage pattern (also same weight and gauge) with a sock yarn "replacement", using the weight as a guide for materials and it came out fine, with some to spare (don't know if I will make matching socks yet, though!). I was a bit concerned with this new pattern, as the called out Marriner's is a wool, and I am planning on using a non-wool blend.
I worked backwards from my last creation, figured the yardage I used, applied that to what the vintage yarn yardage would have been (this was an american pattern, so easier to find the info) and I came up very close, actually under the called for yardage in the pattern. So my tension/gauge is right on for the sock yarn vs. a comparable vintage 3-ply. So I think I will be ok.
It took 6 mos. for me to finish the last pattern on the tiny needles, so having my shop hold the yarn for that long is probably not an option.
It is good to know that UK yarns don't have yardage amounts, so I can tell my sis (also a vintage knit fan!) what to look for and how to calculate.
Glad to see I was on the right track, thanks again for all the suggestions and help! There's a lot of great info on this forum!