Assuming the following criteria:

Head office based in the UK

Purchasing via an in-country reseller 

In-country employees will collect and sign for authentication token (max 50 mile trip)

International employees assumes 5 EMEA office locations

Token weight 50 grams

Long haul flight from outsourced production factory (typically in Asia/China) to manufacturer in US

Long haul flight from manufacturer (US) to UK-based distributor

Road freight from airport via distributor and reseller to end user is no more than 250 miles

Note: Manufacturing and disposal carbon emissions have not been calculated as information is not readily available

Data Sources

Air travel carbon emissions

http://www.tc.gc.ca/civilaviation/International/ICAO/committee/pdf/information/CAEP7_IP10.pdf

http://www.airbus.com/en/aircraftfamilies/a330a340/a340-300/specifications.html

http://www.co2balance.com

Vehicle carbon emissions

http://www.publications.parliament.uk/pa/cm200304/cmhansrd/vo040708/text/40708w05.htm

Carbon absorbed naturally by a tree

http://www.co2balance.com  (carbon absorption rate 900kg for a 50 year tree life)

Trees per acre

http://www.teachingkate.org/forest.htm