Getting a place on the MT degree courses at the top vocational colleges is massively competitive but their graduates have a much higher chance of getting professional work afterwards as all the top agents attend their final shows and make offers to the students. It's almost impossible to get work without a good agent.
The best preparation for applying to these colleges is generally recognised to be the BTEC level 3 extended diploma in Performing Arts (Musical Theatre) which is equivalent to 3 A levels. Make sure to choose a school/ college with a good reputation for this course as quality can be very variable. The course when taught well is a rigorous training in singing, acting and dance plus audition technique. Choose a course run by teachers with solid experience in the industry. It is sometimes possible to study another A level alongside the BTEC but as this is the equivalent of taking on 4 A levels should only be attempted by those of high academic ability.
The BTEC gives a thorough practical based training which can't be matched by performing arts A levels which take a much more theoretical academic based approach as do drama degrees at standard universities.
Entry to the vocational colleges for degree level is by audition only. Though there are no specific requirements for previous qualifications , it is tough enough getting a place for those who have studied the BTEC full time so there is practically no chance of getting a place for those without a similarly rigorous preparation. Raw talent without thorough training just won't do it.
Even the best prepared BTEC candidates from the top schools may take 3 or 4 years or more of auditions to gain a place at the best vocational colleges, many never do. It is very unusual to get a place on the degree courses straight from sixth form as the level of maturity needed to cope with the course means most getting a place are in their early twenties. Younger candidates may get an offer of a year long foundation course to get them up to the level required for the degree course. At a good college these are well worth it as a high percentage go on to the degree course at the end of the year.
The best vocational colleges include Arts Ed, Mountview, Guildhall, Central, GSA, Rose Bruford, LAMDA. The key is choosing one that gets the top agents attending the end of course show.
Happy to discuss further with anyone who wants more advice. DD has just finished in MT at BRIT School and is going on to foundation at Mountview next year. I'm sure she would be happy to advise your DC as she knows the audition process inside out 