The standard procedure in the case of an accident like this is for the insurer to appoint an assessor to determine the cost of repairing the vehicle and submit a report to them. If his report determines that the cost of repairs will exceed the vehicle's market value (as set down in the Argus Cote) then the insurer will 'write-off' the vehicle and offer the Argus Cote value in compensation. The standard Argus value is adjustable up or down to take into account the mileage, certain optional extras fitted and any recent repairs which may have enhanced the vehicle's value. In practice, as this is a pre-determined fixed offer, it will be normally made by the assessor on behalf of the insurer, to whom any dispute should be addressed.
If your insurance company did not specifically require you to disclose the fact that it was a RHD vehicle at the time of the initial proposal and there is no specific exclusion or limiting condition mentioned in the policy terms, then there would be no reason for the standard Argus valuation not to apply. Note that there is no provision in the Argus Cote system to 'down-value' a right hand drive vehicle.
Rather than leaping in to argue for a UK valuation (which is likely to be somewhat lower than a French one), I'd be inclined to wait for the insurer's/assessor's report.
Your can work out the Argus Value of your car with this on-line calculator