The stamps feature the Sun, Venus, Mars, Saturn, Lutetia (an asteroid) and Titan (Saturn's largest moon)
A friend sent a postcard to be postmarked with a special cancellation mark from Royal Mail
on the first day of issue with the Star (a village on the Isle of Anglesey) postmark. This was then sent to me in an envelope...
although the postmark on the envelope did not have as much care taken on stamping it.
Other special postmarks are available. Up and coming postmarks can be seen in Royal Mail's Postmark Bulletin, and a non-Royal Mail website where there is news and information about British stamps is http://www.norvic-philatelics.co.uk/ - well worth checking out, and Norvic's blog (link on that website, and in my blog list).