Is it a lovely dress? Definitely!
Does it look like a wedding dress? Not really. Just a white dress, of which there are plenty.
But what matters is whether YOU like it.
Fair enough if you want an understated look. But If your worry is that you don't want to seem extravagant for a registry office wedding then rest assured, it's your day. You can wear what you want, a full ballgown even!
The only quibble is of course you're presumably having a registry office wedding to save money. If you're canny you can find a 'proper' dress for under £300, sale, ex-display etc.
But again. Completely your decision.