The Germans did make an attempt at producing an artillery device capable of bombarding London. It was called the V-3, and was a massively complex weapon, which used multiple propellant charges 'staged' along the main barrel to accelerate the shells to the speeds necessary to achieve long ranges. These guns were to be housed in two massive bunkers in the Pas de Calais, with construction beginning on the first, at Mimoyecques, in 1943. However, the construction work was spotted by Allied reconnaissance aircraft, and the bunker became a target. Believing it to be a V-2 launch site, the Allies launched several bombing missions, culminating in one on the 6th July 1944, when RAF Lancasters destroyed the complex using Tallboy bombs. Attacks continued after this, as the Allies were unaware of their success - one such attack, using the American Aphrodite drone/missile (a B-24 or B-17 filled with explosives, and fitted with a remote control) failed shortly after launching and killed Joseph P. Kennedy, JFK's brother.

The V-3 was a way of continuing the bombardment of Britain after the Allies had managed to achieve air superiority over Britain and France. Without this, they would never have been constructed. There were several difficult to surmount problems with the guns - the muzzle velocity could never reach the desired range without a serious redesign of the shell, and flash from the initial charge tended to cause premature detonation of later charges. The guns were inflexible, in a way that bombers weren't. The guns, being built into a hillside, had one target, and couldn't be redirected without a serious building effort. In comparison, a bomber squadron could hit targets anywhere within range of an airbase. This included targets further from Calais than London, including the major industrial cities of the West Midlands and Manchester, or the ports of Liverpool and Bristol. The guns had seriously limited accuracy. They could hit London, but hitting a specific target within the city was impossible (where a specific target could be as vague as north of the Thames). The shells had a seriously limited explosive capacity, containing only 25kg of high explosive. The typical German bomb contained over five times as much. A squadron of bombers could carry more explosive, over a greater distance, placing it with greater accuracy, than an artillery piece.

Artillery that large would have been stationary and quite an easy target for a bombing run by the RAF.

An aerial attack such as that shown in The Battle of Britain can be preserved by keeping your planes in an easily defensible position, out of reach from most retaliatory aerial raids.

You must also remember that it wasn't just London that was subjected to air-raids; Manchester and Liverpool were more important targets to the Luftwaffe as the industrial towns held many armaments facilities and refineries. Liverpool was as much as three times the distance from Calais to London.