British Orienteering membership (and HALO’s) runs from 1st January until 31st December regardless of the date you join. (e.g. if you join on May 15th 2015 your membership will expire on 31st December 2015). Membership of BOF and HALO is £10 per annum for adults. You can get even better value for money by joining between now and the end of December. Your membership will last until 31st December 2016. What a bargain.
The easiest way to join is online via the British Orienteering web-site (link to BO website).