I have these, which might occasionally come up on ebay but not at the moment.
Buy All Pro Weight Adjustable Ankle Weights, Blue | 5, 10 & 20 lbs | Vitality Medical
They seem to have been discontinued and replaced with these, which would be phenomenally expensive even if they were available in the UK.
The Cuff Ankle & Wrist Weight | Health and Care
There's very little out there over 5kg per leg. You might be able to use two sets together with a bit of ingenuity, velcro or stitching, but that might not be at all feasible and obviously there is a risk of injury if you get it wrong so you'd have to proceed with caution.
It's frustrating because it is such a simple, accessible programme and so easy to fit in at home - once you know it well you can do it while watching undemanding television, though you do need to do it carefully and check form regularly.
The Strong Women Stay Slim programme just uses dumbbells and substitutes squats and leg raises for the weighted exercises in SWSY. It's definitely worth doing if you can't get the ankle weights, or doing with the lighter weights.
I haven't done it consistently for 20 years, but it is something I come back to because it works for me and fits in with my lifestyle. I do a lot of walking and cycling, then do strength and flexibility work in the evenings. What I love about Nelson is her brilliant understanding of older women and the way she makes it so accessible. Good luck with it, you'll enjoy it.