How can I treat period cottage wooden beams that are around 300 years old?

This entirely depends on their current state. If they are painted you then have two options: strip them using a dedicated paint stripper / remover and then use a beeswax polish to show off their natural beauty or repaint them to rejuvenate the finish. If re-painting, it should be noted that you should only paint over sound coatings and not paint that is flaking or peeling.

Bare wood can be treated with a beeswax polish or a nourishing wood oil.