Were they lithium ion batteries? See:
http://www.britishairways.com/cms/gl...Items_List.pdf
Lithium ion batteries, with a Watt-hour rating exceeding 100 Wh but not exceeding 160 Wh for consumer electronic devices. No more than two spare batteries may be carried in carry-on baggage only. These batteries must be individually protected to prevent short circuits. Equipment containing such batteries may be in checked or carry-on baggage.
Or maybe the security guard was confused by this restriction.