Heathrow to Euston Station

Don't get an Advance ticket unless you build in LOTS of time and are willing to buy a full priced ticket on the day if things screw up.

I'd allow 90 minutes minimum for gate to frosted glass doors, an hour by tube, half an hour for ticket pickup and orientation, and 2 or 3 hours for the possibility of your plane being late or if you have luggage. 5 or 6 all in.

• Taxing, disembark, walk through terminal: 10-20 minutes
• Immigration: 15-50 minutes
• Picking up bags, walk through customs: 5 minutes (the bags are waiting because immigration takes so long)
• Find station, buy ticket, walk to Platform: 10 minutes.

I make that minimum 40 minutes, max 1hour 25 mins. Plus, your flight could be 1 hour late.
Heathrow to Euston, by tube (underground), cheapest option: ~1 hour
Haethrow Express to Paddington, then short tube journey, more expensive: ~40 minutes (wait for train every 15 minutes, train 15 minutes, tube add 10-15 minutes).

Conclusion, what everybody recommends, do not buy a pre-booked ticket for a train immediately after a transatlantic flight, too many variables.
Plan B: book your flight Toronto to Liverpool, buy it as a through booking, then your luggage will be checked through to Liverpool, and if you miss your connecting flight because the first flight is late, you will get a free transfer to the next flight.