Metric ones will fit with some encouragement
This is also an option, but it will take minor tweaking. If you buy
Newark Street metric knobs from Guild, you can most likely adapt them to fit the Westerly Guild pot splines.
Are the shafts on your pots the split knurled kind or the solid kind? I don't have a Westerly Guild, so I don't know. IIRC, they're most likely knurled, split shaft, with 24 fine splines. NS knobs will most likely made for the metric 18 coarse spline shafts. Both shafts have roughly the same diameter 6 mm~0.24 in., but with differing spline count.
As Parker mentions, encouragement will make them fit, kind of like helping a square peg fit into a round hole. Encouragement can mean only slightly drilling out the splines of the NS knob (leave some meat on them for grip) and then filling the holes with a little paper to press fit them on snugly. This doesn't sound elegant, but it works just fine. I've done this before with success.
