either:
Convert avi to mpeg2 using QT or Cleaner, import into DVD st pro, design your DVD, build a VIDEO_TS folder, burn using Toast. Remember to export the audio as an .aiff file at 48Khz and don't include the audio in the video file, this is called a de-muxed file.
Or:
Export avi to QT using the DV stream codec, import to iDVD, design DVD and burn from iDVD (assuming it cab see your drive). The DV file will include your audio and is called a Muxed file.
Everything should be in PAL as you're in the UK
There are other apps that'll author DVDs, but none as good as DVDSP or iDVD.
Have you thought about simply exporting as a VideoCD file from QT and burning that with Toast (burn as a VCD obviously). Some DVD players will play VCDs as well.