The 300nm cross country must be one "flight" with two land aways from the destination aerodrome, done in one day. In other words, as I understand it, you can take off from your home base (say Elstree) fly 300nm as the crow flies (say via Manston, stop Le Touquet, stop Dinard) and that would count. You could return the next day, but the 300 qualifier would be the outbound leg. You can't do 150nm on day one and 150nm on day two.