First of all, your dress.
Find a shop that sells samples and go and try some on. You might find something you love that you can buy straight away, which needs minimal alterations, and you'll probably get it cheaper as well.
Consider also the bridal ranges of "normal" shops like Phase Eight, Monsoon and Ted Baker. These will be more expensive than their regular dresses but cheaper than anything you'll get at a wedding boutique, so again, just order in your normal size and have it altered if need be.
Get this bit sorted soon though, because alterations can take a few weeks.
Secondly, your hen do. It can be anything you like. If you want to make a weekend of it, get an AirBnB somewhere nice, book a couple of activities and go out for a meal and some drinks. If you want to keep it even simpler, just make it one day/evening where you for a nice meal with your friends and then out afterwards if you want to. (Or not, if you don't want to.)
Activity ideas: a dance class (I've done Charleston and Bollywood), a walking tour/ghost walk, a craft activity, an escape room/crystal maze, going to see a play or a musical (I once went to see Chicago in the west end for a hen do), a theme park, medieval jousting (a friend did this on his stag do).
Literally, whatever you and your friends will enjoy.
Don't stress! Do what you want!