In one use of the word 'lineage', Theravada is an ordination lineage. The other two ordination lineages are Mulasarvastivada and Dharmaguptaka. In another sense of the word, you might mean the exact relationships of individual teachers. These are important in the sense that each teacher tends to have their own style and their own emphasis with the teachings of the Dhamma, and it seems to be common that if you appreciate the teachings of a particular teacher you are likely to appreciate the teachings of their students or their teacher, as well.
I don't know where you are in the UK, but there are a number of monasteries in the UK listed at Forest Sangha Monasteries
, which include:
Amaravati Monastery - U.K.
Aruna Ratanagiri Monastery - U.K.
Cittaviveka Monastery - U.K.
Forest Hermitage - U.K.
Hartridge Monastery - U.K.
Perhaps one of those will be close to you. If not, you can find a lot of teaching material provided for free online through those same communities.