if you thinking at the top end the usual route is an undergraduate degree, possibly a masters (but usually in traditional subjects), followed by two years working experience, an mba (american school better though more expensive), and then entry into a higher grade.
so if you are looking at top city jobs and the like i would recommend you get your initial experience and then move on to an mba. most top execs have them and usually two years experience is required. you get on them without but you will struggle.
it would help if you say what you targets are future employment.
as a rough guide if you have the a level points and a 2.1 from a top ten uni you could get into a top tier bb with a pay level of 38k basic. with good bonuses on offer you would be in a position to afford the mba which is incredibly expensive. you might get an employer to pay for it also. your earning potential with a good mba shoots up to approx 75k+ if these areas.
business related degrees are not really overly valued, so what do you mean by a related field? economics yes, business studies no...