The Roman occupation lasted until the 5th century CE, which is 1.5 millenia ago. Prior to that, our knowledge about Iron Age Britain seems to be mostly sourced from Romans, with all the biases they brought in. The Celts (which is a very vague term historically) seem to have been "tribes" and engaged in foreign trade but I'm not entirely sure we have enough information to say they had a "throne" in a meaningful sense, certainly not for a span of 500 years or more (if we generously interpret "thousands" as exactly 2000 years).