The Government considered the use of maximum take-off weight as part of the consultation to reform air passenger duty in January 2008, available at:
http://www.hm-treasury.gov.uk/d/consult_aviation310108.pdf
The Government's response to the consultation, published in November 2008, is available at:
http://www.hm-treasury.gov.uk/d/pbr08_aviationduty_395.pdf