It is true that your guinea pig wold face a long quarantine period if you try to bring him into the UK from the US.
The UK has some pretty severe quarantine laws, and in all honesty this puts most people off bringing their small pets with them. because of this I would be very concerned as to how well they would be cared for whilst in quarantine, simply because this isn't an animal that is often seen there.
The good news is if you fly into Paris (or most other EU or European) cities, you should then be able to take your guinea pig easily into the UK (madness isn't it?).
Guinea pigs do not require a microchip or pet passport, although to make sure you don't have any problems I would try to get a veterinary letter (in Paris) to say that your guinea pigs are in good health and fit to travel. Locating an English speaking vet shouldn't be difficult and you don't need to be totally clear about how long you have actually been in Paris.....
Not sure about US based airlines, but we have flown with both rabbits and guinea pigs as 'cabin baggage' with both British Airways and Swiss.
They need to be in a carrier that will fit under the seat in front and will be secure for the duration of the flight.
But don't let them not being in the cabin put you off.
If you need to fly them as 'cargo' they go in a special, temperature controlled hold, and actually being in the dark and quiet for 10 hours isn't always a bad thing.
I have flown 6 dogs as cargo over the years, and all of them (even the older and more nervous ones) did well. In all cases the captain of the plane was very accommodating about my requests to check the conditions in the hold, and once even showed me photos from the live feed cameras they have there!
In this case you simply inform the airline once you have booked your tickets and tell them you will be bringing pets. I always checked the dogs myself at the airport and was allowed to have them with me until the last moment.
I stayed outside the plane to watch them loaded (even if staff tell you you must board, the plane isn't leaving until your pets are loaded and secured) and once on board I always asked to speak with the captain to remind him they were there (this should be noted in the flight log, but I wasn't prepared to take any chances.
Good luck with it all.
It can be a lot of work and quite stressful, but it is possible.
