Hey,
How many hours are you intending to book this for? You really need to know that people can attend and will attend before you book stuff. If not it's gonna be a very expensive gym session for a couple of people.
The problem with saying you pick the date is that people probably wont. And if there is no date then people wont say yes or no. I can help you this much tho. There is a Lincoln gym on the 7th of march and a Norwich meet probs on the 21st. Feel free to take the 21st if it works tho.
Usual cost for a gym meet is a fiver. If it's much more then it may not be worth it. But depends on the amount of time also.
Only other thing I would add is that I dont think anybody really knows you :/ you are more likely to get people going if people have met you a couple of times before