I use East Coast despite not living there and often not travelling anywhere near there, because of how easy it is to make bike reservations and check the availability of them on lots of different trains.
I also take a careful look at connection times in the journey and don't pick a train with a very tight one in case I can't get off and onto another platform in time with the bike.
Have all your tickets really handy. Getting through barriers with a bike is problematic enough without having to root through luggage/pockets for tickets. You need the actual ticket, seat reservation and cycle reservation one as well because you can get asked for all of them as you pass through the barriers.
I dump large baggage right next to the train while I am loading the bike on so they can't leave the station, then leg it down the train whilst inches away from it (or even touching it) , again so they can't drive off with my bike on board and not me.
Book a seat in the quiet carriage on FGW so you are as close to the guards van as possible.
Ask platform staff which way round the train is coming in so you can be ready at the right end of the platform.
FGW will go nuts if they see you lock your bike in their train. When the train comes in to a platform keep a close watch out of the window that no one without a bike goes near the guards van door. Be ready to give chase and rugby tackle anyone who tries to wheel your bike off the train.
Take a bungee to secure your bike with so it doesn't move around.
Consider separating luggage into valuable and not so valuable panniers /bags so you only need to take one of them off if there's space to leave them on.
Take ear buds in case some really annoying fecker decides to really annoy you in some way.
Have a great trip