I’d echo a few things said above. 8 is much easier to manage and run than 20+ so I’d not see that as a bad thing, as long as the core group are in.
Re. Money. Set a hard deadline to receive it and let people know that if they don’t send you the money by then they will not be getting a place to stay. That £22 figure would be rounded to £25 to be safe as well. Tell everyone today that you’re booking on Friday so if you’ve not got the cash in your account by 9am Friday they won’t be in. Don’t be afriad of using the stag to give people a nudge if needed either.
Re. Activities, same approach with the money as the above, tell them when you’re booking, add 10% to the cost and no money, no go. Also, don’t try and squeeze too much in, the one’s I’ve been to work best when there was 1 activity per day planned, normally starting early afternoon. Then have a table booked for food a little earlier than you’d normally do so people have time to aim for, then ideally have entry to a club organised too. Make sure you have a few bars in mind as you can never guarantee you’ll get into the first one.
Personally, I’d never use one of the companies above but then I do manage people for a living so find this sort of thing reasonably stress free.