Yes I'd go for it, I would recommend having what you are familiar with. Although in my first marathon I was so desperate for anything having ran out of gels, I picked up an unopened one of the ground and ate that, then a piece of something that was either break or cake (i don't know which) that a marshal was giving out. Classy!!!
Unless you one of those carb-free keto-adapted people who can run an ultra on an egg or a pot of cream for breakfast... but you'd know if you are haha.
Yes the changing marquee is easy to find, just the culture shock of all these "normal people" about! First thing to do is change out of the sports bra, it's bliss to get that off after a marathon haha! Then I wear the tee-shirt - probably the only time I'd wear it out of the house given it's as big as a tent on me
Look-UK (the Look Federation) also has places left for a fairly low pledge.
Been debating with myself whether to go for a place this year. Am so tempted as have run the last few years and loved it, highlight of the year. But given that people have sponsored me for various races for the past few years I don't think I could ask for donations so I'd have to cough up most of it myself!!!