if you know where to look there are a number of seats in standard that are not at all cramped; and £70 for an off-peak return is not an outrageous price for a walk-up ticket (the anytime ticket is, however, ridiculous - unless you qualify for a railcard!).

If you can specify a time well in advance, and stick to it, then a first-class advance ticket can be good value.

Taking luggage from LHR to Euston would be quite a nuisance compared to flying - and MAN has an easy rail link to the city centre.