Man up! Haha.
"You really don't know what you can do until you try." - Dr Seuss.
You'd be surprised how open people can be to ideas. Just make sure you dress well and bring something with you for reference (such as a photo of the costume). Ask to speak to the manager, and fairly plainly tell them that you would love to use an area of their pub to do a photoshoot.
Gloss lots of praise about how it's the perfect setting, fluff their ego, and make sure to ask them to decide on a time that would be the most practical for them. You might find that they'll let you in at 9am before they open, and you might find that they won't let you do the shoot at 10pm Saturday night.