The ZX81 came in kit form and also ready built. I had the kit because we couldnt afford the ready built one plus my Dad is an electronics wiz.
I believe the ZX80 was ready built only.
Strange how the topic of conversation has changed so wildly from #1 post.
Both were available as kits if you wanted to save a little money ... These days they'd probably charge you more for the kit than the built item.
LOL ... And I just remembered, WHSmiths was the biggest computer retailer back then too.