I'm in same boat as you, want to do medicine or engineering whichever gives the most money/easiest employment and i doubt 6 A levels will be manageable as theres a much higher work load and i don't know how many lessons you have in a school day but i doubt they will be able to timetable you for 6 different A levels. Also medicine only counts Maths and Further Maths as one and they only take your best 3 or 4 anyway so it won't really matter.Also, They do take into account now that you are more previliged than others to be able to do 6 A levels and go to a private sixth form and so overall it probably won't make much difference