Firstly start with your budget. What can you save? What do you feel comfortable spending? Go with the lower of those options. Add 10-15% to the final number, are you happy with that (saving and spending)? If not, drop your budget to allow for that 10-15% to be your upper limit. I’m going to use even numbers here for ease, this isn’t a suggestion as to what to spend - if you can save £20K and are happy to spend that, your actual budget would be £23K to allow for contingencies and unexpected expenses. If that isn’t comfortable for you, then look at dropping the budget to £15k, with contingencies it’s just over £17K but you were happy with £20K so you say your contingencies budget is £3K.
Then work out your priorities. What do you want from the day. What does your partner want. Do this independently to each other and you each get 3. It could be anything - the dress, the venue, the first dance, the table decorations, the cake, who is there, who isn’t there. When you both have 3, compare and see what you have. Hopefully you don’t have any conflicting ones such as you want a wedding of 300 people with all your family and friends and they want an intimate affair with a handful of people. If you have conflicting priorities, discuss this. Discuss why they want a handful of people and you want 300 people. Is any of this based on a perception that can be altered later (anything less than 300 people and it won’t be a good wedding)? Work on a compromise.
When you conflicting priorities have been compromised on and agreed on and with your other priorities, you know where to split your budget. Start getting some rough prices for the items you want. Don’t book yet. See how much budget you have left. Things like venue will take a chunk of the budget, even if they aren’t a priority. However, you make peace with the fact that something else is more important and therefore you have a more basic venue. We got married at a Best Western hotel and the venue (inc food) cost us just under £2.5K. The best western worked well with our priorities for having a cheap hotel nearby as many people would be travelling. However, if one of our priorities had been a pretty venue, then we’d have looked at other places.
Draw up your short list of venues. Start visiting them. Typically you would get your venue booked first but if your priority is a particular photographer, you pick a date and venue around their availability. If your priority is to have your best friend from Australia attend, check with them that they can make the date before booking. You do need to book the venue pretty early in the process, even if it’s not a priority but then you start going through your priorities and working your way down. If cake isn’t important to either of you and you’ve run out of budget before sorting a cake, you buy a cheap and cheerful cake or go without. Don’t force yourself beyond your budget, especially for the fluff neither of you care about.