Defence - defense - offence - offense
In Britain, always offence and defence.
The 's' spelling (offense) is American English. The adjectives offensive and defensive are always spelled with an 's' in both dialects of English, as is the adjective meaning 'capable of being defended' (used both literally and figuratively) defensible.
Note that in both offence/offense and defence/defense there is a difference in pronunciation in particular the stress: in UK English, the second syllable is clearly stressed, and the first un-stressed. In American English, the first syllable usually has the stronger stress - though for some speakers, the stress is fairly even.