Ahem, as I do not actually need to buy rail fares (waits to be slagged off) I should promote my companies web site so redspottedhanky.com
What time are you leaving and from which station ?
https://www.tfl.gov.uk/cdn/static/cms/documents/london-rail-and-tube-services-map.pdf
They are, if I were you I would go to Victoria then Victoria line all the way the Seven Sisters as opposed to via St Pancras which is very slow