It's difficult to guess how much a fine might be. The court is obliged to take your income into account but normally a court would also be imposing a community order. If the court regarded it as impractical to do so, the fine would probably be somewhat higher.
For sentencing guidelines, see here:
http://sentencingcoun...ine_-_Crown_Court.pdf
Matters dealt with in the magistrates courts can often be sorted out within a few days when the offender pleads guilty. Otherwise it might take a month or two. Cases going before the Crown Court normally take at least 6 months before anything substantial happens.
However the courts would be unlikely to expect you to stay away from your family for that long. I suggest contacting the police (possibly via the British embassy or consulate) to notify them of when you'll be arriving in the UK and inviting them to speak to you. If you do that it's likely that a court would probably accept that you'd return from France for a court hearing, so you'd be permitted to return home while on bail. (If you simply arrive unannounced at the airport they might just assume that you were trying to sneak into the UK, rather than coming back to 'face the music').
Chris