which way you go depends on which Saturday, and how much luggage you're carrying. check here to see which lines are subject to closures over the next few weeks:-
https://www.tfl.gov.uk/forms/12392.aspx
taking the tube to Paddington (for heathrow express/heathrow connect) either requires a walk to Euston Square, or a change at oxford circus; if you instead go tube all the way, go northern line to Leicester square then Piccadilly line (as the transfer tunnel from the Victoria line at green park is quite long). if your train stops at Watford junction, alight there and take the 724 airbus direct to heathrow in 55mn.