You'll be pleased to hear that these tickets are generally available. Price depends on distance from London, but the generic term is 'Day Return Travelcard'.

The cheapest version is valid for initial departure after 9:30am, but they also exist (at a premium) for travel anytime within a day. They are valid on most tubes and buses (everything in Central London included). Also, some of the train companies are offering '2 for 1' admission (or discounts) to many London attractions. Booklets are available at most train stations.

You don't have to book in advance - just turn up and purchase on the day.

Good travelling, Alan in Toronto