Still airfields in the UK providing ATC in uncontrolled airspace. Historic oddity is all I can say.
The answer to the original question is that, in the absence of any local procedures/deemed separations, the departure cannot be released until the contoller has the beacon traffic in sight and can fulfill the reqirements for reduced separation in the vicinity of an ATCO.
Chilli monster and Running Dog proffer solutions which could be argued as geographical separations - provided they keep to the constant or increasing criteria - but they're not provable under the basic procedures in the Boys book of Air Traffic pt 1.
Then again, back in the year dot, the first separation I was ever taught (at Manchester) was "go on, they'll never hit each other". Ah, nostalgia!