The Sinclair ZX81 was the successor to the ZX80, still based on the Zilog Z80A but with 8K ROM rather than 4K, and the same 1K RAM. The biggest technical difference was the integration of many of the ICs into one component, which reduced the chip count and the cost, starting at £49.95 for a kit, and £69.95 for a ready-built machine. It was available in WH Smith, which is where I bought mine in 1982. It eventualy sold over 1.5 million units and spawned a market for many periperals including 16k RAM packs which extended the coding possibilities extensively.
The ZX81 in my collection is not the one I bought in 1982. This, sadly, was sold without my knowing by my parents when clearing our their loft. I found this one on eBay in 2013.