For piecing, I agree with Maniac, a nice medium gray. For quilting if you are doing all Stitch in the ditch I highly recommend a clear monofilament like Superior Monopoly, YLI Wonder Invisible thread or Fil Tec Essence. Not sure which one would be available to you in the UK. Modern clear monofilament thread is not the "fishing line" it used to be. It is super fine, as fine as a human hair. It does not break easily and when it does, it does not create a pokey. If you are dead set against a monofilament, I recommend a super fine thread like silk or Wonderfil (Canadian company) Invisifil in white. The thread is so fine it sinks right into the fabric so if you slip out of the ditch, which is very easy to do, it won't show unless you get your nose a few centimeters away from the quilt.