Compensation is determined for each flight based on its own circumstances. The xEU origination is irrelevant in the case of a BA-operated flight.
Presuming that the cancellation was not due to extraordinary circumstances, you will be able to claim EC 261/2004 compensation of EUR 600 for the cancellation. If the cancellation was for extraordinary reasons, you will not.
While BA could theoretically insist that you fly to your "final ticketed destination", e.g. AMS, in order to claim compensation, that is extraordinarily unlikely.
I am almost certain that BA will, on request cancel the LCY-AMS segment for you under the circumstances.
While BA ought to rebook that segment to a later time, nobody here can tell you with any certainty that BA will not later maintain that if that flight in 3 weeks arrives on schedule, that you are still entitled to the EUR 600. Seems wrong, but you have to assess the risk.