The exchange rate may be an immediate factor, but I think the simple fact is that despite the spending increase that politicians will happily remind us all about, the increase comes on the back of how many years worth of decreases and an overall budget that is insufficient to meet the UK's political aspirations. Dress it up how you like, but governments of all hues have underinvested and overstretched Defence terribly since the Wall came down and the fallacy of their peace dividend was revealed.
But if BREXIT goes badly, watch the exchange rate tank even further and this will not likely be the last immediate cut.