London to Edinburgh via Alnwick

There are no direct trains to Alnmouth from London. You would have to change at York or Newcastle and do a side trip, return.
Newcastle would be the quickest.
Alnmouth does not appear to have luggage storage.
Newcastle does.

I visited Alnwick Castle in May 2023. While I enjoyed it, I would not go out of the way to visit it. Some things were quite gimmicky, no photos allowed in the castle itself as the family still lives there and uses all the public rooms. The garden (separate entry cost) was really nice and I enjoyed the tour of the Poison Garden....

BUT....

I'd probably not go to all that trouble to see it unless someone is a diehard HP fan.

Thanks all - I have the train tickets (via LNER) - arriving Almouth at about 11 and departing to Edinburgh about 5. and ready to take bus or taxi to the castle. It's the castle and the book store we want to see. But luggage....we're stumped on that....which is a significant problem. The castle won't store luggage due to security reasons and the train station doesn't have the facilities.

UPDATE: I ended up just changing my tickets to continue on to Edinburgh and we'll do a day trip from there somewhere once we're settled....

What day trips do folks recommend?

Lin C is incorrect, there ARE direct London to Alnmouth trains- 7 a day northbound and 9 southbound.
There is an hourly bus from the station into Alnwick, also a bus from Berwick station.
So you could easily day trip from Edinburgh by train.
Alnwick also has app based luggage storage options but the decision is made now.

EDIT - Alnwick Castle also contains the Regimental Museum of the former Northumberland Fusiliers, which is free to enter with a Castle ticket

What is the name pf the luggage app you mentioned?

The one I looked at quickly earlier was Bounce, which has two locations in Alnwick.
But there may be others with a bit more looking. These luggage apps are a crowded market.