Prices seem reasonable, considering the place is in downtown Berlin. I might go for the on-site parking, or I might fly, not sure yet (it's an eight hour drive for me...) but I deffo like the idea of it being in Berlin. Dates are far enough ahead that I can plan, this should be OK.
Once we have the group of people penned in, we can discuss group discounts. i think 20 people is ambitious, especially given some people's tendency to say "yes, I'll come" and then ditching you towards the end. If you have paid an up-front charge and a few people don't show, that's your money gone. So we want to be a bit cautious with that.
I do like that you've picked a place and a date without too much hassle. I can tell you're used to wrangling small children efficiently. Which is, essentially what this is, except less diaper changing. Well done you.
I don't think we should orient our budget in GBP though, by July those are going to be worth less than 1€ and it will badly skew the price expectations of those of us not from the Land of Leaving. Let's try and do the math in Euros