Considering high altitude performance while carrying a given payload, wouldn't Canberra be able to climb higher with a wing of same area but higher aspect ratio? Why was Canberra built with a relatively large chord?
The Canberra had its formal origins in a 1944 requirement issued by the Air Ministry for a successor to the de Havilland Mosquito. This requirement, the initial revision being E.3/45, sought a high-altitude, high-speed bomber, which was to be equipped with no defensive armament.
The relationship with the Mosquito followed in its development:
During the early stages of design, the aircraft had grown from being roughly the same size as the Mosquito to being around double its weight.[clarification needed] Although jet-powered, the Canberra design philosophy was very much in the Mosquito mould, providing room for a substantial bomb load, fitting two of the most powerful engines available, and wrapping it in the most compact and aerodynamic package possible, an example being a leading edge formed of a single sheet of light alloy wrapped around to 40% of chord, sitting on Redux-bonded stiffeners through which the ribs were passed, the panels secured with adjustable eye-bolts, enabling a highly accurate wing profile to be maintained from the leading edge to main spar without any external joints or fastenings. Also in line with the Mosquito philosophy, the Canberra by design dispensed with defensive armament, which had historically proven unequal to fighter aircraft, and the resulting performance gain permitted the Canberra to avoid air-to-air combat entirely.
As to its high altitude capabilities, a Canberra B.2 set a world altitude record, flying at 63,668 ft (19,406 m) on May 4, 1953.
Being a bomber, the added efficiency of a larger aspect ratio wing was not seen as a necessity. While it may have added to its range, the longer wings would add structural weight that would likely reduce bomb loads.
But to your question, Martin was building the aircraft in the US under license where it was designated the B-57 Canberra, including several RB-57s used for reconnaissance. That eventually led to the RB-57D:
The RB-57D was built strictly as a high-altitude reconnaissance aircraft. It originated in a December 1952 USAF study funded by the Wright Air Development Center for a turbojet-powered special reconnaissance aircraft with a radius of 2,000 nautical miles (3,700 km; 2,300 mi) that could operate at altitudes of 65,000 feet (20,000 m). Subsonic performance was considered to be acceptable and it was felt that no defensive armament would be needed. The RB-57D was unique and set the stage for high-altitude reconnaissance operations in the rarefied air of the stratosphere. Preliminary specifications were prepared by the Air Force on 27 March 1953. The project was carried out in high secrecy. It was known as Weapon System MX-2147, and the code name was Bald Eagle.
The basic B-57 Canberra design was used as a starting point for the D model, but there were several significant changes incorporated. The most obvious was the greatly enlarged wing. The wingspan was lengthened by 42 feet (13 m) to 106 feet (32 m) overall. The chord of the wing was also increased, which combined with the increased length gave the wing a very high lift capability.
New higher performance J57 engines and new lightweight wing construction to keep the weight down helped provide the enhanced performance. While they performed for a number of years, eventually, there was a need to upgrade resulting in the RB-57F:
The RB-57F was the result of an Air Force Big Safari requirement for a high-altitude reconnaissance platform with better performance than the existing and similar RB-57D, some of which had been grounded as a result of wing spar failures.
The prototype RB-57Fs incorporated many major changes from the RB-57D, the most obvious of which was an enlarged computer-designed wing to enable it to operate at extreme altitudes. The wing had a span of more than 122 feet (37 m), which was 16 feet (4.9 m) longer than the RB-57D and nearly twice the length of the B-57B fuselage on which it was installed. Extensive use was made of aluminum honeycomb wing panels in the wings that bonded outer and inner aluminum skins to a honeycomb of aluminum and fiberglass.[N 3] All control surfaces had tightly sealed gaps in order to reduce drag, and there were no wing flaps. In addition, the size of the empennage was redesigned so that the vertical stabilizer had nearly twice the area of that of the standard B-57B. Its height was increased to 19 feet (5.8 m) and the width increased, improving longitudinal and asymmetric control for greater stability at very high altitudes (up to 80,000 feet (24,000 m)).
The RB-57F also included an upgrade to P&W TF-33-P-11 turbofans, which had more than double the thrust of the original J65 engines. Four aircraft were configured as WB-57F high altitude weather aircraft. Three remain in service with NASA.
Everything in aircraft design is a compromise. The original Canberra did what it was required to do. It evolved from the Mosquito with the engines that were available at the time.
Longer and higher aspect wings were added later to support new missions, but required new wing manufacturing techniques and higher power engines. And that process was repeated again. So, yes, it could benefit from the higher aspect ratio wing and did - eventually.