I will back up what most posters are saying here.

FR will nickel-and-dime you (as we say here in the States) for having the audacity to carry more than a couple days' clothing, check-in at the airport, or (gasp) pay with a credit card.

In 2004, we spent 3 weeks "over there" - arriving London, touring there, flying STN-TXL on Air Berlin (very nice!), spending two weeks on the continent, ending in Salzburg. To connect with BA's LHR-JFK, I booked us on FR SZG-STN, with car rental STN-LHR. Our bags were 2-3x overweight, and I'm sure what we paid to bring our dirty laundry with us, plus the car rental and fuel and overnight hotel required, far outweighed the "extra" we would have paid for a through-ticket SZG-JFK.

Don't do it.