I think funkster is worried about the interview not the tests.
Funkster, since you seem to have a good justification for avoiding flying, ie, wanting to save up to do it all in one go, you may get away with it, although it may seem dodgy that you paid for 5 hours simply to prove your interest then stopped. BA will no doubt read it as they please, however, even if you try to explain it - they accused me of lack of motivation when I had over 30 hours!