Wedding dress alteration stress.

I’ve got my dress from Wed2b but I am really worried it can’t be altered. The dress is a Uk size 14 and I’m a UK size 8-10 more an 8. It’s crepe, lace material and sheath body. I have an appointment with a bridal seamstress but it’s not until the end of August and I’m stressing it can’t be done. I was told by Wed2b that it’s possible the they recommended the seamstress and said they’re highly experienced with their dresses.