Still common to see, popular during the 1970's and 1980's when regular boilers were bigger and heavier. They were installed in a fireplace opening with a fire attached to the front. They are discontinued now.
By far the most popular back boiler was a BAXI Bermuda as they were very reliable and cost effective. There was also a condensing version which due to the cost never really took off.
A back boiler, like regular boilers, need a hot water cylinder and cold water fill tanks (header tank) to supply hot water and top up the heating system. Usually and easily converted to a combi, although converting to a regular is possible.