First we can be reasonably sure that the "prime" is a "bonus" and not just "frais de demenagement" as they are not treated in the same way by the Taxman (see http://info.caprelo.com/blog/bid/91982/Don-t-Confuse-Reimbur...

There are various conditions attached to it being available or not, e.g.:
The Requirements
The employee’s reason for relocation must be for one of the following reasons:-

• they are starting a new job
• there has been a change in their employment duties
• there has been a change of the place where their duties are usually performed

The expenses and benefits paid to the employee must be in at least one of six categories:-

• the employee’s sale of their former home
• the purchase of a new home
• transportation of the employee’s belongings to their new home
• associated travel and subsistence costs
• domestic goods for the new home
• bridging loans in relation to the purchase of the new home

Change of Sole or Main Residence
The employee does not have to sell their home for the relief to qualify, however they must change their residence to the new home in order to qualify.

Time Limit
The expenses must be incurred or the benefit given to the employee within one year following the end of the tax year in which the change to their employment occurred. For example, if a company moved its premises from London to Reading in June 2009, you would have until 5 April 2011 to take advantage of the allowance.
etc...

At this site one can get an idea of the French version, the "prime de rideau" for servicemen: http://www.auservicedumilitaire.com/?page_id=50

Another french ref:
PRIME D’INSTALLATION
La prime d’installation – ou ladite prime de rideau – offre à la personne expatriée un certain confort en termes financier lors de son arrivée. S’élevant généralement à un ou deux salaires, elle permet de couvrir les petites dépenses liées à l’emménagement. Dans le cadre d’un VIE, la prime d’installation peut aussi se traduire par la prise en charge d’une partie du déménagement, à savoir le financement de 150kg de bagages en soute. Dans certains cas, l’entreprise d’envoi peut accepter de prendre en charge l’intégralité de votre déménagement. Ce critère doit être négocié avec votre employeur.
Ref: http://bwing.fr/primes-dexpatriation/