I've been helping NUG leaders globally on getting their Nintex User Group started. I'm glad to know you're interested in starting a NUG in the UK! Please read Nintex User Group Leader Responsibilities and Benefits and submit your information here. Happy to help you along the way to get started on Meetup.com!
I know I am in Canada, but here is a suggestion for you for your user group.
A new Nintex User Group has been started in Toronto Canada. I would love to attend, but due to location and travel time I would have to take a half day off of work.
Think about the ability to either do a Podcast, Skype or WebEx session of part of the meeting (say you have a keynote speaker come in for a presentation) or all of the meeting.
Or have a Q & A session during the meeting and open it up to people on line and select a few questions to answer during the meeting. The rest of the on line questions can be answered later before the next meeting or have other members pick up a question to respond too.
Good luck with your user group and all the best.
We have a few running in region here in AU/NZ also using Meetup.
Chris Ben is running New Zealand - https://www.meetup.com/en-AU/Nintex-User-Group-New-Zealand and Martin Harris Thomas Xu are running Perth - https://www.meetup.com/en-AU/Nintex-User-Group-Perth if you want to reach out and pick their brains. We're planning on kicking off in other cities in 2017.