Does the earliest we know of Britain only date to 8th century myths? Why was Britain so primitive at that time, relatively not long ago, compared to other nations with rich records of history like the Caliphates, China, and India?

Firstly, there are Roman records of Britain (though not all reliable: for example, they claimed the British performed human sacrifices prior to Romanisation, which might be true, or might be propaganda to justify Roman imperialism. Still, they do give quite a bit of reasonable information about Britain), so it dates back to the 1st century. As to why Britain was so "primitive" at the time, well it was essentially at the far end of the known world. It was a backwater, far away from the trading lines that went from China and Northern India through the Middle East to the Meditteranean. So knowledge was not disseminated by trade routes the way it was in the Middle East. It also had a fairly small population.

Also, consider that Britain in the 1st century BC was not the exception. Rome, the eastern Meditteranean, northern India, Persia and the Han were exception, with cities, scholars, and so forth. Most of the world was inhabited by tribal groups of illiterate farmers or hunters, just like Britain.

Anyway, if we look at Britain in the 8th century, well again it was pretty average, with small kingdoms and a small number of educated people. It's Byzantium, the Caliphate and China that are unusually advanced, with their high populations and position at either end of the silk road (or in the middle). One might as well ask why Sweden, Ireland, Russia, Gambia, Argentina, Angola, Mongolia, Canada, Sri Lanka, Jamaica, Indonesia, Australia or Cambodia were primitive in the 8th century. Because they weren't one of the most advanced places in the world. Actually, 8th century Britain wasn't so bad. It's population was reasonable, it had trade, cultural and educational links to a wider European (Christian) culture, and a reasonably literate caste of educated specialists who rarely tried to overthrow the political ruler. The main issue it had was the way Vikings (from poorer, less educated societies) kept attacking. I'd say overall it was neither very "advanced" or very "primitive".

Of course, we then come to the issue of what exactly is "primitive", and can cultures really be described this way? It kind of assumes that societies naturally "advance" but this isn't necessarily true. And frequently societies can be "advanced" in some ways and "primitive" in others.

Anyway, to summarise, Britain was at the far, far end of trade routes, and had a small population, so wasn't so well incorperated into trade networks that brought new technology and ideas. Contrast this position in Antiquity and the Early Middle Ages with the 16th century, where the discovery of the Americas switched Britain from being at the far end of trade networks with little ability to exploit trade, to being in a perfect position to exploit new markets in a new, heavily naval, economic paradigm.