The only occasion I can remember the opponent's anthem not being booed or whistled at by England supporters was against Lichtenstein.
And there's a good reason for that!
Yeah, like all the Bulgarians who attended the last home match v England have now died off.
Even if they had done, they would still have had the chance to tell their kids about it - as if they didn't know about it already, because (a large minority of) England fans have been notorious for it...