It is acceptable to season your food in a restaurant although you must try a couple of bites before doing so. When asking for either the salt or pepper, both should be passed to you and placed in front of you rather than in your hands. Salt and pepper cellar/mills should travel around the table together, like a married couple never to be parted! If the salt and pepper are served in cellars then an appropriate amount should be poured on to the side of your plate and the food dipped into them, rather than poured across your food. If they are served in mills then they may be ground across your food.
See more etiquette advice at www.emmadupontetiquette.co.uk/
To add a nutritional aspect to the previous answer regarding the salt:
In 2003, Scientific Advisory Committee on Nutrition published a report, where they concluded, that there is a strong link between high salt intake and high blood pressure. It was also concluded, that salt intake in British population is dangerously high and the action is required to reduce it.
As a result, salt reduction was made one of the public health priorities by the Food Standard Agency. Food Industry was encouraged to reduce salt content voluntarily in over 80 food categories. The companies, that agreed to support this programme included Costa, KFC, MacDonald's, Pizza Hut and others. These companies reduced amount of salt in their products according to guidelines. To find more about this initiative, check out the FSA report.
So, not going into further details, the reason your food is not salty enough might be because this particular restaurant aims to adhere to these guidelines. Therefore, the decision to add or not to add salt is entirely up to the consumer. Feel free to add salt, just try not to exceed daily requirements of 6 g a day! :)