We almost did - we had a blessing here, and would've had it really cheap (£500 for 75), but PIL stepped in...
I'm assuming you're talking about just the reception? We were going to rent a hall (you can actually get nice spaces, try your council for old buildings, etc), and get nice food from Waitrose. It'd have been more work, but I was excited, and thought it'd be really elegant. IF you're only having 20, you could prob find restaurants willing to hire out a room. We actually did much of our US wedding really cheaply - and did DIY flowers (went to the supermarkets the day before and bought what was pretty and fit with our colours - everyone loved the potted tulips, and they made great favours!), and had the wedding party pitch in a bit, so we weren't paying as much for service.
Are you thinking of photography and clothes as well? Monsoon is great for dresses, and if you're worried about money, just ask for the guests to all take pictures, and send you doubles? (Some of our favourite pictures were actually non-professional.)