If it is a Yucca--many other plants mistaken for it--the flower clusters come out of the center of the leaf rosettes. If the trunk is 6 feet tall already, it is more likely to be a Cordyline australis, since the taller Yuccas either aren't hardy, or won't grow in such a wet climate as the UK. The "puds" on the stem are probably new branches starting, in response to the Arctic frosts last winter. When Cordylines bloom, the fluffy clusters of tiny flowers also come out of the centers of the leaf rosettes. The frosts have made blooming somewhat unlikely this year, as similar frosts did last year. Blooming is more likely with some sun, and with yearly feeding with something like growmore.