Having read a lot of the blurb and seen the talks too, my understanding is that there's no point creating an engine without having a feasible concept spaceplane to stick it on otherwise how would you know what the engine should look like, how big it should be, whether it could actually get something into space?
Did Mr Whittle have a similar point of view?
I don't know but he probably had at least some idea of what his engine would be required for. Whether it would fly fast or high or for long or may not have been all that critical as long as it could make something fly at all.
SABRE has to get a vehicle into space with enough payload to matter and that is a much more demanding requirement. Already they have gone through 4 engine designs because their models using Skylon showed them what the trade-offs would be. If one used a significantly different vehicle then I presume that some of those trade-offs would change and force yet another design to be done.
They have certainly made statements in the past about it being difficult to scale down the engines without making them uneconomic. I don't know if that's still valid but it's the reason they have given for not making a smaller vehicle to test the whole concept.