Hi Lyton,
Welcome to VirginFlyer.
With the notable exception of VS3/4, which is operating the Suites-fitted aircraft, Virgin don't put a particular aircraft on any specific routes or publish their operations schedule in advance. The best (and only) prediction is available on this site in the 'Which Aircraft' section. Here is a link directly to the LHR-IAD route:
http://www.virginflyer.com/chance.asp?flight=VS021As you can see, nearly half the time in the last three months, they've used a A340-600 (and for some reason, V-GOGE was used nearly all the time in October). However in the last month the pattern has been pretty mixed between 747's & Airbuses (Airbusi?) - so probably one of the most difficult to predict.
You can normally get an indication of the aircraft scheduled to be on the flight if you call the Flying Club a couple of days before your flight - although there is always the rider that the aircraft can change at any time (literally right up until the point it takes off).
Pix.