Go-Below is highly recommended. Try them first, they will give you a great experience.
If the afore-mentioned cannot accommodate you though, all the Outdoor Centres in N Wales are geared up to take groups u/g and specialise working with children and adults alike.
They will all have a variety of mines they can use that have been signed off by the relevant Mines Engineer as being suitable for group use.
Oaklands Centre, Marle Hall, Llanrug and The Kent Centre to name but a few of the centres. There are many more.
PM me if you get stuck.
If it's raining, put a mountain on your head