The three V-bombers were all originally built for the high-altitude bombing role. In 1952, while the Vulcan was performing its first flights, the Air Ministry requested preliminary concepts for a "Low Altitude Bomber (LAB)" to complement the high-altitude bomber force, with the LAB penetrating enemy defenses at an altitude of about 150 meters (500 feet) at close to Mach 1.
General concepts were submitted by most of Britain's aircraft manufacturers. The Avro proposal was the "Avro 721", and was in the form of a canard machine with a small, cranked-delta wing. Powerplant options included four projected Napier NP.172 turbofans or two Rolls-Royce Conway turbofans. The small wing made getting off the runway with a full warload troublesome, and so built-in rocket boosters were considered. The bomb was to be ejected backwards through doors in the rear.
The LAB concepts submitted by the manufacturers were given a looking-over; meetings were conducted in 1954 on moving to a full requirement specifying an operational aircraft, but the studies indicated that a LAB would require considerable effort and involve high risk. The aircraft demanded a high strength-to-weight ratio, new navigation systems, new weapons and associated delivery schemes, and so on. The cost of development was estimated as high as 15 million pounds -- a modest sum for development of a combat aircraft these days, even adjusting for inflation, but a major expense for those days, and more than the budget could bear. The conclusion was to not seek an operational system.
Since the V-bombers ended up operating at low level anyway, the decision not to proceed comes across as a lost opportunity, but LAB really was pushing the state of the art. The Royal Navy did obtain a low-altitude tactical bomber in the early 1960s, the superlative Blackburn Buccaneer, which the RAF would adopt as well -- though only because all alternatives got the axe. The RAF was not happy about the Buccaneer at first, but once in service it proved an outstanding combat aircraft.
