Having tickets to and from London is not an issue, the only requirement is being able to leave the Schengen area. The problem is that to apply in the UK, you need to be a resident, as specified on the embassy's website:
You must be resident in the UK
Your UK residency must have at least 3 months validity after the intended date of departure from the Schengen area.
As per the decree 2008-1176 of the 13th of November 2008, the French Consulate, London, is not competent in processing visa applications of people not resident in the UK, i.e. holders of ’C-Visit’ visas or of a British visa valid for 6 months or less. People who are not resident in the UK must apply for their Schengen visas at the French Consulate in their country of residence.
If you were a resident of the UK, you wouldn't be expected to return anywhere else. But since you are not a resident, your application will most likely be rejected (i.e. the consulate will decline to examine it; your fee would be refunded and there would be no record of any refusal).