When dealing with customer service, deal with the one issue which matters, e.g., being rerouted on your preferred date of travel. Asking about compensation is asking the wrong person at the wrong time and does not remotely solve your problem.

There are innumerable flights on innumerable carriers between IAD and LHR (not to mention WAS and LON) on any given day. I cannot believe that none have space. Determine what works best for you, call VS and note that your scheduled flight was cancelled and that you need a reroute on the date of travel. Ask for the specific carrier and flight # you want and you will most likely get it. Additionally, there are a good close to 50 flights/day between WAS and NYC (including IAD-JFK if that's what you really want) and Amtrak is under 3 hours on the Acela, with onward flights on VS. Consider that as well.

Finally, if you want it, you are entitled to a full refund of the cancelled segment. VS can tell you what that will be. Could be that you can locate a single of your own which suits your needs. If that's the case, take the refund and purchase. Or, if your trip has not commenced, cancel the ticket and purchase what you want.

Schedule changes, including eliminating flights are common. Luckily, there are few city pairs in the world with as many options.

As to EC 261/2004, it applies, but provides for no compensation as the notification was made at 14+ days. It does, however require VS to rebook you under substantially comparable conditions. If VS had the only weekly service, this would be a problem. But, we're talking about the capital cities of two major nations. I would not worry about alternatives.