Maybe, but how much has been spent on subsequent ballots on striking instead of voting on the offer? It may well be a pathetic excuse from RDG/DfT, but it is equally so from ASLEF. They know what they need to do, and they can choose, or not to do it. Personally if this were my union, I'd be a bit peeved that my union was blocking a possible move forward in negotiations. Just to reiterate, RDG/DfT are not interested if ASLEF members want to strike, they want to know if the membership officially accepts or rejects the offer. That is the position as it stands.
And again, none of this worries the government. Those increasing amounts of the outstanding backpay are sitting in their coffers, and the government could still withdraw it altogether. If that happens, then ASLEF will have to start all over again with a new government who will have to re-cost backdated deals, potentially in the light of being in a very difficult financial background. A sweet deal from Labour, which I suspect ASLEF are now holding out for, may not be nailed on.