By Jamie Spencer Manchester United have been dubbed the most valuable club in world football with a total estimated worth of $6.2bn, ahead of Real Madrid, Barcelona and domestic rivals Liverpool. Sir Jim Ratcliffe agreed to purchase an initial 25% for $1.3bn last December, increasing to 29% over time for an additional $300m. That deal valued the whole club at $5.5bn, which suggests his investment may already be profitable – not that he’s in it for the money, by his own admission. Madrid are valued at $6.06bn, Barcelona at $5.28bn and Liverpool at $5.11bn. The presence of Manchester City (sixth…