Dont worry about what anyone else thinks. You are doing it for the two of you..and you should have the wedding you want, where ever and however you like.
Registrars see couples coming in by themelves alot, sometimes staff at the registry office are the witnesses! There is nothing wrong with a small wedding for the two of you.
If you do want to have just the two of you, talk the registry office, and ask if they would be able to provide witnesses.
Otherwise, are there a couple of close friends or family you could ask?