Hey, I'm so sorry that you and your fiance are going through this, my thoughts are with you.
You can absolutely do this!
If you get married in a registry office with around that many guests, you're probable looking at around £300-£450 depending on room size and your location. For reception, if you want a party maybe look at hiring a village or sports hall and decorating it yourself? If you aren't fussed about a party type reception, maybe book a meal at a nice restaurant?
Also, check out Groupon - they often have great deals for weddings. Additionally, some of the venue suppliers here on Hitched (and most venues) offer last minute deals to use up dates, so have a look around - if you're flexible on dates, you can probably pick up a great deal!
Good luck, my thoughts are with you. X