This information is not held centrally and could be provided only at disproportionate cost. In 2013-14, for example, the Ministry of Defence (MOD) paid more than 2,700 different contractors. We would need to examine the work of each one to establish whether it could be considered a 'private military company'.
The MOD publishes a range of trade, industry and contract statistics, including a list of suppliers who the MOD paid more than £5 million in each financial year. The latest information covering 2013-14 is available on the gov.uk website at: https://www.gov.uk/government/ statistics/mod-industry-trade-and-contracts-2014.