We got married in 2019 in central London. The whole day cost £12.5K but we could have spent A LOT less. We splurged on a nice venue with great food for 50 guests. Plenty of free booze. That amounted to £8.5K alone.
The other £4K was on registry office fees, a routemaster to get all guests from RO to venue, Prosecco for drinking on the bus, my dress (£200 off the peg), shoes, his suit and shoes, flowers from the market, fairy lights for the venue, taxis, invitations, cake ( M&S plain tiers decorated by me), hotel for nights before and after, Magician for the drinks reception, evening snacks.
We utilised all our contacts to the full- we're friends with tons of creatives so got mates rates or freebies on photography, music etc. We decorated the venue ourselves and cleared up ourselves. I made my own tiara, wedding bouquet, table decorations, jewellery, did my own make up and hair, made paper flowers to decorate the cake with... if we'd paid market rate for a lot of this stuff we could easily have hit the £20K mark
I'm a florist and he's a DJ so those spends were very low. 
The things that saved us money were getting married in winter, buying clothes off the peg, not using professional photographers, doing the music ourselves, buying cheap cake and doing our own flowers. Invitations were £100 from Vistaprint, I made the seating plan for £20 with cardboard from Hobbycraft and printed off at home.
The most important thing for both of us was that our guests were happy, well fed and not bored. We spent 20 minutes tops on photos, made sure there was money behind the bar all night and had an absolute blast.