###Edited: added details about the Interpol SLTD database and what they say about using a passport declared lost, and about emergency passports.
The form used to declare the loss of an identity card or passport (Cerfa 14011*02) has this big red box at the top:
Any declaration of loss or theft renders the ID definitively invalid. If you find it, you have to bring it to the authority which delivered it and not make use of it in any case.
The official government website service-public.fr also states:
Une carte d'identité ou un passeport déclaré perdu sera invalidé et ne pourra plus être utilisé
An ID card or a passport declared lost will be invalidated and will no longer be usable.
In theory, a passport or ID card declared lost should have been entered into a database of lost or stolen IDs, which should be available to the authorities and possibly to foreign authorities via Interpol's SLTD. If you use such an ID and they find out, not only will the ID be invalid, but you might get into quite some trouble to prove that you are the person represented in that ID, and not someone who stole it.
###Advice for travellers
Do not attempt to travel with a document that you have reported as lost or stolen.
Once you have declared your travel document as lost or stolen to your national authorities, it is cancelled and considered invalid. The details of the document are passed on to INTERPOL and entered into the SLTD database. Border officials in INTERPOL's member countries can screen passenger information directly against the SLTD database. Selected airlines can submit the document details through I-Checkit for screening.
If you try to travel with an invalid document, entry or boarding is denied. The travel document is seized to prevent its future use and you cannot travel.
Whether that actually happens (i.e. whether the passport was entered into the database, whether the French border police (PAF) actually have access to it and check it, and whether the UK border police have access to it and check it) is anyone's guess, but you should probably not try it. The PAF often just have a quick cursory glance at French IDs and could thus miss the fact that it's reported lost, but the UK border police will nearly certainly check. If they have access to the information, you will be in trouble.
Best case, she could go back to the place where she filed the form (the police) and ask them if they did indeed file the information into the database (or if the form just went to "vertical filing" aka the trash can). Maybe she was lucky and they just didn't do anything with it. But I pretty much doubt it. IIRC at some point only large-scale thefts of blank IDs were recorded, but that has probably changed by now.
Note that even if there was a procedure to revert the declaration (officially there isn't), it would probably take a long time to take effect everywhere, if at all.
The form also states:
Cette déclaration ne vaut pas document d’identité.
This form is not an ID document
So you cannot use it instead of a valid ID document, only use it to get new ID. And even if you could use it as a temporary replacement in an emergency, it would most definitely be useless 4 months later, no police force will seriously consider it.
If your girlfriend has a national ID card (carte d'identité, CNI), then she should use that.
Otherwise, she should apply for a new passport and/or ID card and you should postpone your trip.
Note that she could apply for a temporary emergency passport (same-day delivery), but the rules to get one are quite strict (medical or family emergency, work emergency...). "I forgot to ask for a new passport to go to London for the week-end" does not quite qualify, but YMMV...