In public, an officer carrying out an ordinary stop and search can only ask you to remove your outer coat, jacket and gloves. They cannot require you to take off more than that in public view.
If a more thorough search is needed (for example removing a t-shirt, or any search involving more than outer clothing), it must be done out of public view — such as in a police van or a nearby building — and not in a police station unless certain conditions apply.
Any search that involves exposing intimate parts of the body has stricter rules again and must be carried out by an officer of the same sex, away from anyone of the opposite sex. These "more thorough" and intimate searches are tightly controlled and are not part of a normal street search.
The search should be carried out with courtesy, consideration and respect, and should take no longer than is reasonably necessary.
This is a plain-English summary. For the exact rules, read the official PACE Code A.