Been doing a little more checking
The National Rail journey planner allows you a notional time for crossing London:
10 minutes at each "main line" station (the same as if you were changing between two NR trains there, Warrington-Euston-Wembley Central for example)
plus 23 minutes for Euston-Charing Cross or 28 mins for London Bridge (the times you found on the NR site), which allows for the additional walking time to and from the tube, the travel time and waiting time.
So Euston arrival platform to Charing Cross main line station entrance allows you 33 minutes, then another 10 minutes to find your train. This is pretty generous (even from coach A at the back of your Virgin Pendolino!) and you might well make the 1445 as you suggest.
BTW - follow signs for Northern Line: Charing Cross Branch - Southbound, and any train will take you to Charing Cross.
PS Why NR thinks the Charing Cross branch of the Northern Line only runs every 15 minutes, I haven't the foggiest.
PPS In the worst case scenario, if BOTH branches of the Northern line are disrupted, you have a third option: take any Victoria line train southbound to Oxford Circus, walk about 20m through a passageway (it's about in the middle of the platform IIRC) and straight on to the Bakerloo line southbound to Charing Cross. Takes about a minute longer than the direct route! But you shouldn't need to worry about that....