I used to only travel in 1st class on Avanti (with my senior railcard, and splitting, it was affordable) but stopped about a year ago due to the product becoming increasingly diminished and inconsistent. I tried Standard a couple of times, and that was bearable but not how I like to travel, so I've settled on Standard Premium, which is turning out to be exactly what I want. It's often quieter than 1st class, and you don't have the staff constantly (and noisily) bustling up and down the aisle. With a carry-on meal from M&S or Pret, I don't miss first class at all.
I saved a very significant amount of money by splitting my first class ticket, as the fare for my full route is set by Crosscountry, whose first class fares are criminal, but I notice the OP lives in Carlisle and thus unlikely to find similar savings.