There is a certain amount of confusion about the classification of businesses that offer accommodation due to constantly changing government regulation. In recent years the requirements to be able to call yourself a hotel have been made more onerous, resulting many small hotels being forced to reclassify themselves as guesthouses instead. However even hotels that achieve the requirements of legislation will often advertise a "Bed and Breakfast" rate in order to compete at the budget end of the market, even if full-board accommodation and extra facilities for guests are available.
Private House Stays lists only those accommodation providers who are small enough for it to be worthwhile for the owners/managers to display the availability of each of their rooms. Thus there are few businesses that have more than ten rooms listed. However the Association believes in that there is much value to be had for customers in creating a stronger market presence for 'small is beautiful' establishments because there has been an increasing trend for staff in hotels to be foreign workers and impersonal. There is usually the opportunity to be able to talk with owners of small businesses to get their recommendations and advice about local attractions and facilities, when hotel staff have no knowledge or opinion.
There are many thousands of Bed and Breakfasts across the UK, some that are registered as such, and some that just call themselves this when they are private houses accepting paying guests. The difference merely that a you would need to be a registered Bed and Breakfast in order to have a sign outside, which would imply an acceptance that customers might ring the doorbell to ask for room availability. There is no need to be concerned if you have booked with a Bed and Breakfast without any visible sign outside - it is a choice made by many businesses that choose only to accept customers who have pre-booked. The term Bed and Breakfast does not pre-clude that lunches or evening meals would not be available by arrangement, particularly in country-side areas. It is merely a generic term that would imply that guests would usually be expected to vacate their rooms for the majority of the day and that there might be limited facilities for guests outside of a bedroom and a breakfast in the morning. Since a good proportion of Bed and Breakfasts are run by their owners with limited staff, they may not be contactable by telephone at all times and you should expect to have to inform the owners/managers of your estimated arrival time.
There is not much difference between a Guesthouse and a Bed and Breakfast other than an implied larger and more professionally run establishment. A guesthouse is more likely to have staff, a 24-hour reception and better facilities for guests than a Bed and Breakfast.
In remote locations especially there are plenty of Inns and Public Houses that have rooms available on a Bed and Breakfast basis. Indeed they may be the only options for food and drink for miles around and can have very nice accommodation available. The only down-side to staying in an Inn or Pub is that you may not have complete quiet until after midnight and the bar has closed.