I have taken a small backpack on numerous Euro gigs, especially during 2010/11 and never had a problem.

Seems there may be some slack regarding other aspects also. It is normally a given that you are not allowed to take in a DSLR camera with detachable lenses. I have it in writing from the Hyde Park organisers that you can now take a DSLR if the lens is less than 6inches long. So I might take mine considering I have DC for that one and will have no need for a bigger telephoto lens.