Have you switched mobile data off in settings? If this doesn't work and you want to remove an insert SIM message you could get a PAYG SIM and then switch off data.
I've watched movies while mine is plugged in and it still seems to charge slowly.
Previous Phones: HTC Google G1, HTC Hero, Nexus One, Nexus S, HTC Desire HD, Samsung Galaxy SII, HTC Sensation, Galaxy Nexus, HTC One X, Nexus 4, HTC One, Nexus 5, Sony Xperia Z2, Nexus 5.
Previous Tablets: Motorola Xoom, Nexus 7.
Current: Nexus 5, Sony Tablet Z LTE.