Sorry, i'm not london based, and i'm an old married, having got married in 2008. But we were planning our wedding on a tight budget.
We kept our cost down by having a village hall for our reception venue, it wasn't the flashiest of venues, but it looked lovely once we'd decorated it. We were also then able to bring in our own caterers rather than using some that were part of a venue package.
We managed to get a 5 course wedding breakfast for 80 people at £15 a head. It wasn't fancy food, just your standard soup/melon - roast - desert - cheese and biscuits - tea/coffee, but there was lots of it and it was good food. Everyone commented on how it was nice to have something simple but delicious.
I forgot to add, we did our whole wedding for £5k. That included church, wedding cars, photographer, venue, food for 80 for the wedding breakfast and 120 in the evening, suit hire, stationery, cake, decorations, buying bridesmaids outfits.