There are a few offers dotted around the UK Amex Membership Rewards web site:

East Coast trains – 20% additional value – £25 voucher earns £30 of travel.
House of Fraser – Extra £10 on a £100 redemption and extra £20 in a £200 redemption
Dyson DC34 bagless vacuum cleaner – free set of tools with a redemption of 38,450
Samsung 40″ TV – free Blu-ray player for 153,880 points
Lathwaite’s wine – 2 free bottles with a redemption

You can also use points for Amex Gift Cards:

£20 4,500
£30 6,750
£50 11,000
£100 22,000