cake - they sell plain white ones in supermarkets for about £10 and pretty ribbon is a few pence a metre - if you want a topper get one off ebay or get a small posy of flowers
flowers - market or supermarket and tie up with same pretty ribbon - by us you can get bunches of lilys or roses on the market for about £4, or gorgeous gladioli are only £2 in Asda, tied up with ribbon they are quite dramatic
photos - can you ask your guests to email you all the photos they take on the day, with a bit of creativity (cropping etc) you could make a nice album, and then get any you want printed done at Asda (70p each)
venue - I know you said the registry office was grim, but is there a local park or any green space that you can walk to, or someone's garden
dress - charity shops have some gorgeous ones, and you can at least try them on, ebay might be a bit 'pot-luck' with this. Ditto grooms suit and any bridesmaids dresses
food - do you know anyone with a costco card or similar?
music - that's what phones are for isn't it, make a soppy playlist and off you go
Have a great day, congratulations Little