Sunday Evening Arrival in London

You would certainly have better selection than on the train, probably availability of better quality as well.

I love a train picnic and train stations usually have good choices.

https://www.networkrail.co.uk/communities/passengers/station-retail-directory/?station_id=5632

Here is the site where you can find food options at Waverley for your journey.
M and S always has a good selection of sandwiches and salads.

I hope your seats are on the left hand side of the train traveling forward for the best views!

absolutely the best idea. If you drink you can bring a small bottle along too...

Maybe see if you can get a table rather than airline seating?

Totally I would stock up before boarding the train.

A far better range- even at Marks and Spencer prices cheaper than the train and a far wider selection.

Or there is a Sainsburys in the Waverley Mall (right outside the station)- their 'Meal Deal' is very good value- £3.50 for sandwich, snack and non-alcoholic bottle/canned drink. And add on a bottle of wine (or whisky) if you want!!

Wetherspoons are a UK pub chain. While often snobby opinions about Wetherspoons are mixed, having a late breakfast or early lunch may not be a bad idea if you have time.

You never know if the catering crew will turn up, or what stock levels will be. The other week I was on a peak evening train out of Euston to Glasgow on Avanti (I know you are on LNER) and all hot food (and almost all sandwiches) had been sold by the time we passed Milton Keynes, half an hour into the journey. The cupboard was almost bare. Glad I had stocked up.

Also I may have been unlucky but I've been on several services recently where card machines have been out of order- so cash only. Doesn't affect me, as I always have cash, but could anyone who is relying on their card.

Absolutely get some food and drink to enjoy on the train. Crisps, fruit, sandwich, and bottled water.

Thanks for the suggestions! Our train departs at 10:00, so we will have time for a good breakfast. We will plan on taking food on board, as it is nearly a 6 hour trip due to maintenance on the usual route, which means we will be heading down to Carlisle and then across to Newcastle and then down.