Strangers in the Night

Start the evening by agreeing on a time and place to meet - in this case, a popular book cafe in London. Arrive separately, perhaps wearing something a little different from your usual styles. You don't know each other, remember?
Inside the book cafe, maintain the pretense of being strangers. Catch each other's eye, maybe even pretend to bump into each other accidentally. Slowly build up the chemistry, sparking up a 'stranger's conversation', and allow the evening to naturally progress.
As you both enjoy the evening, perhaps Gosia 'accidentally' reveals that she is 'cheating' on her 'boyfriend' with you. The thrill of this confession adds an extra layer of excitement to the scenario.

To execute this scenario to perfection, you both should spend some time getting into character before meeting up. Think about what your 'stranger' character would be like. What would they like and dislike? What kind of books would they recommend to a 'stranger' they've just met? This will help set the scene and give you plenty to talk about during your encounter.
Consider dressing slightly differently from how you usually would – maybe even wear a perfume or cologne that you don't typically wear. This will help you both get more engrossed in the roles you are playing.
Lastly, decide on your 'backstory'. What's the reason for this 'affair'? Giving this some thought beforehand will add realism to the evening.

Fully commit to your roles - avoid breaking character whenever possible. This will make the scenario much more exciting and enjoyable.
Feel free to add your own twists and turns to the scenario. The key is to make the evening fun, exciting, and memorable.
Finally, remember that this is just a roleplay scenario. Be open, communicate freely, and respect each other's comfort levels throughout.