Travel Disruption Vouchers CAN be used for web fares.
I had a travel voucher from a delayed train last year that was nearing the end of its validity and went into my local booking office to see if there were any current special offers I might be able to make use of; only to be told I could actually use it against web bookings contrary to what I always believed!
All one has to do is bring in proof of a web booking that has not already taken place (i.e. the ticket) and they will refund the cash value of the voucher against it. I had an upcoming booking on the system so that's what I did.
A pity that this fact is not mentioned ANYWHERE by IE.