Once you have completed your bar finals and been called to the bar you apply for pupillage. That is extremely competetive. I think if you look on the Bar Council's website there are some statistics. I seem to recall that there are something like 16 applications for every pupillage place. Whilst many large chambers will sometimes take on up to 6 pupils a year, they very often take only 1 or 2 tenants on, if that. So even if you successfully complete pupillage (and trust me, that is the most god awful year of your whole life), the competition for a place in chambers is enormous. It helps if you have a specialist area and it particularly helps if you have a first class degree and a grading of outstanding on the bar finals course. Out of the thousands who start bar finals only a very small few make it to a tenancy. Even when you get tenancy the battle is not over since then you need to build up a practise - if you have a good clerk and you are dedicated though, half the battle is won.