Luke,
It really depends on what type of reward you would like to have. If you would like to have cash back, then I would go with the Morgan Stanley Platinum card:
http://www.morganstanleycard.co.uk/uk/data/car d/platinum/plat.html
or, the American Express Platinum Credit Card:
http://www.americanexpress.com/campaigns/affil iates/uk/platinumcredit/platinumcredit.cgi?PID =827&BUID=CCG&PSKU=D7X&CRTV=plckntext&AFFID=12 76495&sourcecode=X49000IE23&tduid=1b8d08673764 915c1b8a2a0c656661b6
I have both of the above cards, but I tend to use the Am Ex when travelling overseas, especially in the US.
However, if you want to get vouchers for retailers, then you could always try M&S' And More card, or John Lewis' and Waitrose's Partnership cards.
I would warn you that the cards that offer the most attractive benefits tend to have an high APR to match. I clear my balance in full every month, not paying a penny in interest, yet still get the benefits they offer.
If you do not clear your balance in full, then I would aim for a card that has a low APR. The below link should help you. You just select what you want from your credit card, and let the search engine to the rest!!
http://www.moneysupermarket.com/cards/credit_c ard.asp
This is where I found the Morgan Stanley card, and the Am Ex card.
I hope that this was useful to you, and good luck with your search!
Scott