You must be absolutely sure of what you want, so that dressmaker can give you an accurate quote - which means you have to try on as many dresses as possible - make a note of:
Material - eg - silk satin, chiffon, organza, duchesse satin

• The shape - eg empire line or strapless etc, etc
• How many layers of fabric? - is there an underskirt?
• Details such as embroidary, type of train or whether it requires a bodice? Bodice work will push up the price - but all ready-to-wear wedding dresses that have bodices are expensive anyway!

Also - always keep your paperwork - dressmakers work with so many brides it is hard for them to keep track of the cost.
 
The fabric makes a big difference to your costs.  London has a fabulous selection of materials. The quality is superior and the fabric shops will help you with getting the right colour for your skin tone.