FD 24-35L is not a replacement to FE 16-35, especially not in the corners. But in the mid frame, lens is quite sharp with excellent contrast, well controlled CA and good sharpness. It represent IMHO great value for an WA all rounder, considering above mentioned IQ aspects, very good build quality and affordable price.
I don’t have experience with FD 300/2.8 L but it would be hard to believe that is on par or even better than EF version which was simply outstanding. I replaced it with EF 200/2 L IS and 400/2.8 L IS II, which combined with Canon EF TC’s are covering all the range that I might ever need. But if I wouldn’t get stunning deal on both of those lenses, I would keep 300 and be happy as well.
£450 might be slightly over, but it will depend on the lens condition… It is indeed very good lens.