Chris,
Can't answer this about Wembley, but I've encountered it elsewhere.
I was on the Lions tour of Oz in 2001. I sat in the stadium in Sydney, minding my own business, filming the the preparations of the test match on a video camera, and was told to stop filming, or have the camera confiscated.
It seems that the filming rights of live events are now sold off and fiercely protected, precluding you from taking video images of the event. Maybe this extends to digital cameras, as they have video capability.
As to what constitutes "professional equipment", I have no idea.
Digital SLR's are common place now. Just because you own a digital SLR, surely it doesn't mean that you earn a living from it, or even know how to use it?
I say take it, and if you get pulled, stop using it.
Best of luck