We were in England this summer and managed to find them easily enough.
There are Accomodation desks at tourist offices and rail stations that will arrange booking at local B&Bs. They will tell you the tarriff and you pay ~£5 to the booking agent and the B&B will take that amount off the rate.
Although 2 of the nicest ones we found were on the recommendation of a local innkeeper (that was fully booked) or pub owner (which belonged to the owner).
One of our favourites in the UK is on the north coast of Scotland at Strathy Point. We stayed there just as she was starting her B&B around 1997. We’d gladly return. We were invited in to dine with her young family for breakfast. They were a delightful family.
B&Bs are still everywhere, particularly in tourist spots.
For those less brave than the OP and don’t like the ‘pot luck’ approach, there’s a great guide to B&Bs published by Alistair Sawday - these are the pick of the best (NOT most expensive, by any means) and include the quaint and quirky. Some may drift into the ‘boutique hotel’ category, but many are traditional old two-room affairs.
We have a quite amusing reality show called ‘3 in a Bed’, where three couples who own B&Bs take it in turns to stay at each other’s places and criticise the bed linen.