Your arrival time at LHR is a little awkward, for onward train connections.

As has already been mentioned, booking an inflexible "Advance" ticket this close to travel date is not going to save you a great amount of money (had you booked months ago, then the saving would have been very significant), so you might want to consider buying an off-peak ticket, which is not tied to a particular train. You could book it for the 14.43 departure, in order to get the reserved seats on that service, and take the earlier one if you get to Euston in time. You should have no difficulty in finding seats on a train at that time of day, even without reservations. You don't even have to book an off-peak ticket in advance - you can get one for the same price at the ticket office at the station (but you would not be able to get seat reservations that close to departure).

However, the off-peak period ends at 1500 (3 pm), after which the flexible fares are exorbitant until about 1900 (7 pm), when they revert to off-peak. You need to consider what you will do if you are delayed, and arrive at Euston after the 1443 train has departed.

You will save money by buying a Two Together railcard (assuming there are two of you), which costs £30 and gives you a 34% discount on train fares when travelling together. You can buy a digital one online.

I don't think the tube is going to be the quickest way to get from Heathrow to Euston. You could look into Elizabeth Line + Northern Line, or Heathrow Express + taxi. A Two Together railcard will also get you a discount on the Elizabeth Line and Heathrow Express (though you will have to buy tickets from a machine - you won't get the discount if you tap in using a bank card).