Hi mate,
There is no right or wrong answer as it soley depends on your true ambitions. If you want to work as a ramp agent but no higher then say so and this is perfectly acceptable. Every applicant will be different. However you may find that when you start off you begin to get higher aspirations and this happens to a lot to people coming in at the bottom, wanting to climb the ladder. However you may not know this untill you've started.
If however you do want to climb the ladder but want to start as a ramp agent to progress your career, don't be afraid to hide the fact.
The trick to it is the way it is phrased.
For example saying this:
"Yeah i'm looking to get into dispatching but can't find any work and don't know enough so thought i'd come and be a ramp agent for a year and doss about for a bit untill a position opens up where i'll be able to use my current status as a spring board"
Say something more like this:
"My true ambition is to get into dispatching or higher management airside work however I don't have the experience or skills so want to start as a ramp agent to start my career.
I looked at the BA site at the job title and responsibilities and I think i'm right in saying that a ramp agent is basically a baggage handler. With the current security issues it may not be a very high staff turnover job but i'd hazard a guess that generally it is. With this in mind BA will budget and anticipate high staff turnovers and know that many of their ramp agents are doing it because it is earning a living rather than having a true desire to do the job. So don't worry about not sounding too interested in doing the job if you want to progress. The truth that you wanting to progress is the important thing to BA.
In the end it is reliant upon where you are, where you want to go and how you're going to get there (classic strategy management).
The easiest way to tackle this is by being as relaxed and as honest as possible. Try not to say what you think they will want to hear as this ofter makes people come unstuck. If you want to progress but need to start as a ramp agent to do this tell them. They should respect your honesty and ambitions. However if you don't want to progress, they should respect that too.
In the end it usually comes down to job interview experience and this will come with time. At the minute the best thing you want to do is stay truthful and tell BA exactly what your intentions are. They may just ear mark you as 'one for the future'
Good luck mate.