This is (primarily) for Brits.
1. Suppose I normally buy 10 apples at $1 each. Then the government taxes me $3, buys 3 apples, and gives me those 3 apples. I will now buy 7 apples instead of 10. The net result on my consumption of apples, and everything else, is zero. The only difference is that the government is doing part of my shopping for me.
Fiscal policy won't work in that example, because I offset it 100%. But this post is not about that example; I only used it to help illustrate my second example:
2. Suppose the government of Canada normally buys 10 new Canadian bridges at $1 billion each. Then the Bank of Canada prints $3 billion, and instead of lending it to the government (buying government bonds) like it normally does, it buys 3 new Canadian bridges. The government will now buy 7 new bridges instead of 10. The net result on building of bridges, and everything else, is zero. The only difference is that the Bank of Canada is doing part of the government's shopping for it. [Update for clarification. Printing the $3B will have an effect, but the Bank of Canada buying bridges instead of buying bonds will have no effect.]
Remember that the government of Canada owns the Bank of Canada, so that any revenue the Bank of Canada earns by printing money and buying bridges or bonds ultimately ends up in the government's pocket anyway. So it's exactly like my first example, in terms of who ultimately pays for the apples or bridges.
If you expand the central bank's power to include buying bridges, we should expect to see 100% fiscal offset for any exercise by the central bank of its new power.