When parents pay for/contribute to weddings there are normally strings attached even if they're not outright spoken. For example, you might have been planning to have a wedding with 40 friends and your immediate family but suddenly your Mum has invited all your relatives or Dad wants his golfing buddies to come and you can't say no because, heck, they're paying for it. So you try to say "oh but there's only room for fifty people" and next thing you know your friends are off the invite list or you're parents have paid for a new venue instead.
Of course, this doesn't always happen, but it does occur a lot. If my parents were very well off, and I was struggling, I might ask them to help me (or take an interest free loan off them- this is a good option so long as you can pay it back). However, if they were not financially well off, or if they had already turned down my older siblings, or if they didn't seem open to the idea (for example, if complained about their friend's children "robbing their friend" to pay for a wedding) I wouldn't dream of it.
If you want to borrow/ask for or accept money from your parents:
Have a clear written agreement
Don't take more than you can pay back
Be prepared to compromise over details about the ceremony.
Don't let it strain your relationship- what is more important, a ceremony or your parents?
Let's say you can pay half (I think you said something about 11.5k) and your parents pay a quarter and your partner's parents pay a quarter. That's a more manageable sum to ask to take or borrow.
That all being said, I don't think you should ask for family help unless they approach you or seem really open to the idea.
21k is a lot, but you can do it for a lot less, particularly if you pull in non-monetary favours (maybe your cousin is a cake decorator, maybe your aunt is best friends with a seamstress, maybe your mum's wedding dress fits you). You can also do a short honeymoon at, say, a country cottage or something. It doesn't have to be a trip to Fiji. You can also "save the honeymoon" maybe for your 5 year anniversary?
Also, if you have any spare cash at all, start a savings account now, even if it is just a spare change jar. That way when you do get married (and I'm guessing it would be at least 2-3 years from now) you'll have something to put towards the ceremony.
Edit: and one more thing, grandparents are often more generous than parents. And stoic, quiet uncles. I'm not saying ask them for money, just that help often comes from unexpected quarters.