I read somewhere, quite a while ago, that if you repeatedly use the same contactless card for tube journeys in excess of a certain amount, then a maximum daily amount kicks in and you get charged that, rather than the number of trips you make. But - if I need to go the London from Southend and I'm expecting to use the tube, then buying a 1-day "travelcard" is, I think, the most economical way of doing this.