Royal Mail shipping rates use the products weight in KG. (Please make sure your settings are in KG and not Lbs)
http://www.royalmail.com/

Configuration Settings
Status:
Zone 1 Countries:
(List comma separated ISO codes)
Zone 1 Handling Fee:
Zone 1 Shiping Rates:
(Comma Separated)
Zone 2 Countries:
Zone 2 Handling Fee:
Zone 2 Shiping Rates:
(Comma Separated)
Zone 3 Countries:
Zone 3 Handling Fee:
Zone 3 Shiping Rates:
(Comma Separated)
Zone 4 Countries:
Zone 4 Handling Fee:
Zone 4 Shiping Rates:
(Comma Separated)
 

Shipping rates are worked out by weight/price.

e.g. 1:1.29 (For a parcel of 1Kg or under the shipping would be £1.29)