Geographically, London isn't a bad place for a city. It's at the lowest crossing point on the main river in southern England, and the river, which is tidal at London and provides excellent water transport to the rest of the country and Europe. It also has a nice mild micro-climate and a fertile hinterland.

London hasn't always been the capital though. Under the Roman Empire various cities served as provincial capitals including Colchester, York and London. In the period immediately after the Roman withdrawal archaeological evidence suggests that London was partially or entirely abandoned and then resettled in the early Anglo-Saxon period. The various kingdoms of the Anglo-Saxons all had their own capitals, none of which was London, and when England was unified in the 10th century, the capital was Winchester, which had been the capital of Wessex, the kingdom which had done the unifying. Winchester remained the capital under the Norman kings. Under their Angevin and Plantagenet successors government departments and administration and the law courts started to settle at Westminister, a separate city (which it still is, legally) about a mile west of mediaeval London, where they have remained. The kings at this period were mobile, moving from around the country and their French territories as needed, but by the late mediaeval period they had settled at Westminster and a network of palaces in the Thames Valley. Parliament too became settled at Westminster. Initially kings summoned it to wherever they happened to be (though most frequently at Westminster, often in Westminster Abbey) but in the 16th century it gained a permanent seat in the royal palace at Westminster. Throughout the middle ages London was the largest and richest city in England and though neither the king or the government lived there, it was where the money was, and so control of it was politically vital.

Why do you consider Paris more sensible than london?