Most cities, towns, and villages have public restrooms, indicated by signposts to the "WC," "toilets," or "public conveniences." They vary hugely in cleanliness. You'll often have to pay a small amount (usually 30p) to enter; a request for payment usually indicates a high standard of cleanliness. Gas stations, called petrol stations, usually have restrooms (to which the above comments also apply). In towns and cities, department stores, hotels, restaurants, and pubs are usually your best bets for reasonable standards of hygiene. The Bathroom Diaries is flush with unsanitized info on restrooms the world over—each one located, reviewed, and rated.
The Bathroom Diaries (www.thebathroomdiaries.com.)