Or was London always a powerful area that was always destined to control the island ?

Well yes, but actually, no.

London is a very conveniently located site for a major urban settlement, and it was a major economic and civic centre during the Early Medieval Period in the centuries preceding the unification of England, but it wasn't 'predestined' by any means. Indeed, the Roman settlement at Londinium was abandoned for several centuries in favour of the English wic of Lundenwic about a mile downstream roughly near the modern site of Covent Garden. It was only in 886 after the reconstruction of the Roman defences and the implementation of a new streetplan and urban layout in response to repeated Danish threats that Lundenwic and Londinium occupied the same site.

London is well situated as an economic and political centre. It's a strategically important crossing of the Thames, and it's ideally located to allow for the confluence of maritime, riverine and land-based trade using the Roman road network. During the 8th and 9th centuries, London was also one of the three most important mint sites in England, alongside Rochester and Chester. London remained politically and economically important throughout the Early Medieval period, but wasn't necessarily 'destined' to become a capital; following a plague in the South East in the 890s, Chester developed rapidly after its restoration in 907 to become a major economic rival, as well as a highly prestigious fortress, trade port and mint site. In Mercia, Tamworth and later Gloucester were important political centres, while in Wessex, the Gewisse centred their dynasty on Winchester, and the Northumbrian kingdoms centred on Eoforwic or York, which thrived further as the Danish settlement of Jorvik.