This is (deliberate) moving of the Overton window. The Guardian was e.g. heavily against Jeremy Corbyn and was actively involved in purging the left wing of Labour. Left-wing/far-left in UK is something like the Morning Star.
Don't assume bad faith when someone expresses their opinion (see rules). When even Wikipedia describes The Guardian's readership as "generally on the mainstream left of British political opinion" (as I quoted in another comment) there is nothing wrong, disingenuous, or inaccurate in saying that The Guardian is left-wing.
Yes, they are generally over the centre line. They are, however, still firmly centre-left, in that The Guardian has opposed every major push towards even mild social democracy. Most of the UK left would find The Guardian's editorial line well to their right.
You're right, I should have talked about the Overton window more generally. But public usage of terminology is part of moving/spreading the window. It's quite apparent in the US discourse when very moderate politicians like US mainstream democrats are characterized as "radical left-wing extremists". Granted, there's a lot of hyperbolical "fascists" and "nazis" thrown around from the other side.
I find mainstream left of UK to be quite clearly center left. The current Labour government policy could be characterized even as center right.
When the person starts out by describing a mainstream paper as left-wing propaganda, I think its fair to be sceptical about the degree of good faith, no?
I think it is your reply that is not in good faith at this point because either you haven't read my comments or you have chosen to ignore them.
Anyway it is getting excessively tiring not to be able to discuss or say anything so have a nice day.