This is an annoying issue, but I'll clarify a bit of it.
FIFA ruling is that no team can edit their kit with a political, religious, company or charity symbol, no matter the cause. The reason is that it will encourage the others to happen, such as a "if they can, why can't we?" attitude.
It's wrong, but the poppies on the armbands is good enough. The poppies also resemble ANY person that has represented the country at war, not just WW1 or WW2.
And Spain will destroy us, for starters Rooney isn't playing, Young is injured, there are two out-of-form centre backs up to start. It's gonna be a battering.