When you order a new dress if it has beading embroidery on the hem it should be provided in the exact length to fit you to avoid costly and very difficult alterations. You will need to wear your wedding shoes for the fitting. Ask if the shop assistant is experienced in measuring and fitting. A lot are not.
If you buy a sale dress that has beading and embroidery on the hem and you are too short for the dress you will need to get a definite quote from the shop for the cost of alteration. You could be looking at well over £100 plus for the hem alone and if you are looking to save money by buying a sale gown you do not want to spent a fortune on having it altered.
A tip here if you are ordering a new dress. When yours arrives examine it very carefully as if you were buying a dress from the rail. Check the inside of the bodice for marks and look under the hem for wear and dirt. Unscrupulous shop owners will try to sell as new their sample of gowns that are not getting many orders. This happens a lot in the bridal industry. If a shop has a sample dress that does not get many repeat orders the shop owner will try to get rid of it to the first person you likes it.