I have personally experienced airspeed fluctuations due to heavy icing whilst going round in the hold in UK at around FL100. The aircraft was in a steady state with a known power set. My (LHS) airspeed fluctuated 50kts or more over a period of 30 seconds whilst IAS 2+3 remained reading the same steady state. I considered executing the UAS abnormal procedure but with traffic in the hold both above & below us I considered doing nothing was the safest option. My only action was to retard the thrust levers to current power & disengage the ATHR to make sure there was no ATHR misbehaviour. After 60-90 seconds we made VMC and all indications went back to normal. The aircraft remained in normal law throughout. During the encounter we picked up serious amounts of icing around the windscreen.