We had a DIY wedding. Although it was in a church.
This is what we did/had:
Booked the wedding with the church/priest (so, the same as you booking the registrar)
The legal bit at the registrars office (can't really remember but I think we needed birth certificates?)
Organist (church wedding, see)
Suit and shoes for him, dress and shoes and hair thing for me (I didn't wear a veil! I had long curly red hair right down to my arse! no way was that going to be tamed). His suit was m&S, my dress was from pronuptia, off the peg but had to be altered to fit (was way too big). It was an ex demo model, but cleaning etc was included in the cost (£300). It was perfect - very plain white satin, not a hint of Xmas cake about it. Not a hint of lace or anything frilly.
Flowers - local florist delivered the basic package to the church
Bridesmaids dresses - we bought them with the grown up bridesmaid (my sister) and DH's sister (mother of his 4 year old niece, who was the flower girl) from somewhere like Laura Ashley or monsoon.
Venue for reception - local labour club - we decorated it with balloons bunting etc the night before.
Music - friend's hi Fi. I did the tapes.
Food - local vegan caterers, they did the wedding cake too. And the drinks.
Invitations - WH Smiths
Photographer - local person, basic package (no video)
Whole thing came in well under £1.5k - possibly under £1k actually.
We had 45 guests. It was fab.