Complex sentence: From 1923 until 1925 McKay served as president of the European Mission, headquartered in Britian, and with direct responsibility over all Church funtions in the British Isles and supervisory funtions over mission presidents on the European Continent.
Split into simpler sentences: From 1923 until 1925 McKay served as president of the church's European Mission, headquartered in Britian. In this capacity he had direct responsibility over all church funtions in the British Isles and supervisory funtions over mission presidents on the European continent.