I can't quite understand the license benefits of the LAPL
That's because there aren't any!
If you can't afford the proper PPL now, then I wouldn't start with it. Flying is an incredibly expensive hobby to keep up, and it's such a shame to see so many people get their licence then let their rating lapse after two years as they just can't afford to stay current.
If the money is really that much of a problem right now, either fly microlights or wait until money isn't a problem before you start.
You're quibbling over £600 now, that's nothing next to the cost of an IR, even failing the test can cost over £2000 in retakes.