Depends what you want to do and how 'formal' you want it.
You could probably find a restaurant that would be happy to provide a meal for 40, for the cost of what you would normally pay (particularly if you were talking January when they are usually quiet rather than pre-Christmas).
If you phone up hotels and mention the word wedding though, prices do tend to get multiplied by at least 3.
Or you could do more of an afternoon tea?
Or, I went to a lovely wedding a few years ago (couple had very little money) where they persuaded a local pub to give them their room for free (as pub would then be taking all the money behind the bar), and asked us all for our order from the local chip shop, who delivered to everyone in the pub room. It was a really, really nice, informal occasion.
Or book your local Church hall / parish rooms / Scout hut / community rooms and get a local caterer in ?
So difficult for people to give you an estimate without knowing how 'traditional' or 'formal' you want to be.
The legal bits will be the same, whatever you do afterwards, but will still need to factor in.
Then, are you buying a bespoke wedding dress, or a 'non-traditional dress' for you to get married in?
What will groom be wearing ?
Will you be having attendants?
Flowers ? (Bouquets but also decorations for room)
photographer?
cake?
transport?
church or registry office or part of hotel package?
Would you feel compelled to pay out for someone else to do your make up / hair / nails / tan or not?
Do you want to send out formal invitations ?
Do you want printed order of services?
'What is realistic' is going to depend on 'what is important to you' and whether you are just 'trying to bring it in under £12k' type budget or whether you really only have £500 to spend. 