At present one ATR is fully utilised doing Mancheste, Rennes, Manchester, Manchester. The second does nothing first thing, Caen on three days, Groningen in the afternoon and nothing in the evening. So there is space for a new double daily route, but like you say there aren't many obvious options and they have an existing option in Rennes that works.
It could be that they're seeing how the new routes go - giving flexibility for GLA for example to be downgraded to the ATR if sales aren't too good