Good thought on no alcohol, SP. You might want to have a quick response if someone asks why you're not drinking in a pub. (Yes, people can be that rude without meaning to be.) You can make a joke, such as, "I'm pregnant." Or, you can say that you want to be clear-headed the next morning for some reason.
Look CUTE. Be charming. NO sarcasm. NO "inside-joke" jabs. Pretend you don't know your W and you're just a guy in a bar with some acquaintances.
It might be difficult to control your non-verbal facial and body language. Try not to distance yourself too much from your W if you really want your friends to believe you're still together. Which leads me to:
Why the heck are you guys going out with friends right now? I think it's stressful to have to pretend you're together. It would be different if you were all going to see a movie or a play or stand-up comedy. But, you're going to a pub. Nothing to do but face each other and talk. Too much pressure. I think I'd get 10 ulcers in that evening.
If you can fake it well, you scare me a little with your master manipulation talents.
Anyway, whatever happens, I'm vibing good stuff for you.