I got married this year after 18 years together with OH and 3 months before 3rd baby was born. Honestly, we should have done it years and years ago. We met very young and marriage was not important and we baulked at the fuss and expense. However once we started having kids it became important to me. Kids have different name to me (when I travel alone with them I have to prove I am the mother), kids were illegitimate (so we had to sort out complex wills)....
We had a fantastic winter (cheaper) wedding, organised in 3 months flat, no fuss, no frills but still had 90 guests with sit down meal and evening dance. We decided what was important and cut out the (to our eyes) unimportant stuff (DJ, wedding car, fancy dress, photo booth, etc). We had good food, good wine, fantastic ceilidh band, did pretty much everything ourselves to keep costs to a minimum. Our biggest expense was the venue (a renovated historic barn, but because we got married in February it was more than half the price of a summer date) and the food (because to my mind you have to receive your guests well, so we went for the waitressesed buffet option, good food still but cuts down staff costs, paper napkins rather than linen, no chair covers...), did everything locally (outfits, food, flowers, photography, etc) to get best deals and for ease of organisation. I hired a photographer, but just for 2 hours and the contract was for 100 photos so paid a fraction of the normal wedding package. His parents did a booze cruise (which they enjoyed) and bought the wine as a wedding gift (we had cava Buck's Fizz rather than champagne on arrival, 2 bottles wine per table, sparkling desert wine for the toasts). Among other things my parents bought loads of chocolate after Xmas in the sales so we had bowls and bowls of lush choccies in the evening. His sister baked our wedding cake instead of a gift, my cousin designed and printed names and seating plan instead of gift, my siblings organised games and song lists for the hour or so between the end of the dance and closing (the other guests were luckier, we said no gifts, just come and join the celebration!). We had an absolute ball, the whole event was relaxed and joyful, and our kids loved being part of it (DD was bridesmaid, DS ringbearer) and called it "our" wedding and keep asking when we will do it again! And we spent what we could afford (£6k) for a still great party rather than blowing £20k as is apparently the average these days. We had no honeymoon and both went back to work 2 days later.
I was wondering what would change in our relationship once we were officially married, and to be honest, with a house, cars, kids, etc. it changed nothing but I am happier because the vows we took sealed our relationship in many ways and brings stability not to me, but to us as a family unit. Have a simple wedding now, and a big party later if you still want it. I have loads of friends who had really simple wedding, like weekday with a pub lunch which are lovely because they are very relaxed. The party is irrelevant, it is the marriage bit that is important.