Call the CP Desk and tell them you are ready to book flights and ask them to check and see if the CP-cert eligable upgrades are available for your specific choices. With the flex dates you've mentioned, you should not have too much trouble (particularly on the DCA-CLT-LGW route).

As for the search for "Preferred Upgrade" fare feature on the US website, I find it basically useless. It works for domestic flights but a lot of the time it just shows the highest unrestriced coach fare out there which is often more than a discounted F seat. Further, I am not sure how well this works for international flights as it will (for example) choose a full-fare booking class even if it is an Express CRJ from DCA to PHL/CLT.