Train reservations—how to reserve forward facing seats

For Eurostar, the website gives a seating map that shows which direction the train is going in. I've also gone to https://www.seat61.com/ - it's a great website for picking out the Eurostar seats, and there's a lot of information for other trains too.

I used that site on my first trip on the Eurostar, I have the same problem with not being able to sit backwards. I actually got my husband and I seats at one of the tables so we'd be across from each other, in case I messed up and picked a seat facing the wrong way, lol.

I don't use apps, but on the LNER website which I have open now, you will be automatically offered a seat- then below that choose 'Manage your seat reservation', you will then get a train plan which at top right shows you the direction of travel. Then choose which car you wish to travel in, then your seat (which should go green), then hit the 'Save your changes' button.
I imagine the app does the same thing.

If I book an LNER (or any other train) on Northern it asks me to choose forward or back, aisle or window, power socket or not, table or not and quiet car or not. Based on those preferences the system then allocates me a relevant seat.

LNER should do the same for other operators- for instance GWR.

Bear in mind a train can end up 'in reverse formation' for various reasons (that is, basically travelling backwards) in which case it all counts for nothing. Your forward seat will then be backwards.

Assuming you aren't traveling solo, if the carriage is equipped with facing seats, you can reserve a pair or more facing one another, and then switch off if necessary.