The plants mentioned are hardy, spacing depends on the plant - things like Ajuga would be on about 33cm centres for it to knit together relatively quickly. If you just want extremely tough no-nonsense ground cover, rather than anything fancy, I would go for Prunus 'Otto Luyken' or Cotoneaster dammeri at around 45cm centres (or both, bearing in mind the Prunus will grow to around 4 ft tall and can be sheared to keep it low, whereas the Cotoneaster is only a few cm tall and hugs the ground).
You can plant them further apart btw but it will take longer for them to knit together and form a continuous ground cover. 4-5 plants per sq. m is what car park landscaping is based on!
"What is hateful to you, do not do to your neighbour".
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You can plant them further apart btw but it will take longer for them to knit together and form a continuous ground cover. 4-5 plants per sq. m is what car park landscaping is based on!