The accents...
Just seen an advert for this...And the scouse accents sound dreadful, especially the one on the advert, she can't roll her r's at all, this sounds petty but thats one of the big differences between a bad scouse impression and a good one!
Why not hire someone who sounds authentic for gods sake?
Secondly, I'm not entirely sure the modern scouse accent is a very good basis for the one which would've been around in the WW1 era...
Pedantic I know, but really I'd expect the BBC to have this kinda stuff covered.