I have applied for a France shengen visa but am not able to go in France. Instead am going to Italy and Austria and then London. so will there be any problem at immigration due to France Schengen visa or while departing outside of the Schengen area?
You may still be able to enter Italy/Austria instead of France. You may need to explain why - tell the truth, lying does not pay off.
If you have reasonable explanation, I'd expect you will be granted entry (maybe with a notice). It's likely to affect your ability to get French visas in future, but should not have huge effect on getting Schengen visas via other countries.
However if you have time, it's best to contact embassy of the main destination country of your trip (Italy or Austria) and ask them what's the best way to proceed. It may be better to apply for another visa, or to get an official letter from the embassy if they say another visa is not required.
As for London, it's in UK and UK is not in Schengen Area, so you will need separate visa for that.