My wedding was on a budget too so I totally get where you're coming from. Then later in life I ended up working in the industry for a while (markups are INSANE)
Date wise was just after a "romantic" "public holiday" so I went into the shops the day/weekend after this and snapped up leftover decorations.
Bit trickier for you as it's just BEFORE the associated event BUT you may be lucky and be able to get Halloween/autumn themed stuff online now fairly cheap.
Re buying the dress great of your mum to help but be good to keep that cost down too. Boutiques sell last season and former display dresses at a discount or if you're open to non traditional styles/colours you could go bridesmaid lines or high street selections. Same applies to veils and shoes. Tbh you're very unlikely to get a bespoke new dress at this late stage anyway.
Very very easy to make your own jewellery and even tiara if you're that way inclined or know someone who is.
Ditch the non essentials like favours, seat covers, centre pieces for tables (honestly once the settings and wine are on there there's little room anyway and guests in their finery "naturally" decorate the venue) as even done cheaply it's an additional cost you simply don't need!
Make your own stationery super easy just printed off or if you do want to go a bit special hand written invites for example using metallic pens or calligraphy if you or someone else has the skill look perfectly acceptable. Hell you could do e-vites!
My cake I kept VERY simple, 2 tiers one plain one fruit so that most people were suited and it was the dessert.
Buffet meal (we had lots of different dietary requirements and allergies to consider anyway, a sit down option would have been impossible!) but it was catered so a served buffet as it were and everyone then got to choose but felt looked after
Only bouquet was my posy, I made satin bags for the bridesmaids and flower girl to carry, the flower girls dress was made/donated by her mother as a gift to us she looked gorgeous!
Hair I had done by my usual hairdresser in a fairly simple style with silk flowers included in the style. Where I cocked up on the hair was I wore a crew neck t-shift to go to the hairdressers and couldn't get it off over the hair safely! Mum had to cut it off me! (Old t-shirt thankfully) I went to her and quite honestly on the day it was nice to have an excuse to get out the house for a bit and have a bit of peace and normalcy!
Make up I did myself (I HATE people touching my face and I hated the idea of the super heavy make up the majority of MUA go for especially for weddings) including my nails which were simply trimmed, filed and painted by me. I can't do false nails, but admittedly they weren't really a thing back then anyway. I bought new make up and nail Polish but my usual brands and liked choosing the colours myself as I knew what suited me.
Rings were from Argos! £75 for both! That's a very personal choice though. Ex and I weren't really fussed about this.
My earrings were also Argos but actually with hindsight I wish I'd made my own as I'd have liked something a bit different
Photographer is tricky as I'd normally say that's the one area you don't want to skimp as that's the most long lasting part of it all! But you can shop around/get recommendations and choose the least number of photos options and don't bother with their album/frames as it's easy enough and quite enjoyable to get your own frames and an album and sort all that out yourself.