See what the weather is....if decent, head for Windsor, a lovely day away so to speak! You could, feasibly, go first to the CCR, leave your carryon with the porter there and pick it up on your way back through. Get breakfast in the arrivals lounge...tidy up a bit and then head to Windsor or just about anywhere else in the London area. Be sure to head back in good time to clear security, pick up your carry on and get to your gate...I'm a little cautious so would plan to get to First Wing at least two hours before your flight...it's easy to kill two hours in the CCR and F lounge. Ahh for the days of a spa! No longer!