The RAF did use 8000lb bombs against cities in Germany. These bombs, and their 4000lb and 12,000lb cousins, were part of a series given the label HC, for high capacity. These bombs were constructed to contain the maximum amount of high explosive, and were colloquially termed 'Blockbusters'. They were a very simple design, little more than a steel cylinder filled with high explosives - typically Amatol and Torpex. The 4000lb HC could be carried by most aircraft of Bomber Command, including the light Mosquito bomber, but the 8000lb and 12,000lb were only carried by the Lancaster bomber. The typical bomb load was one HC bomb, and multiple small incendiary bombs. However, these were not the heaviest bombs dropped by the RAF. To solve the problem of destroying hardened targets, such as sub pens or railway tunnels, Barnes Wallis devised what he called an 'earthquake bomb'. This was a large bomb, dropped from high altitude, that could penetrate deep into the ground, before exploding. The shockwaves from the deep explosion would then destroy the target. Two such bombs were produced, with the designation D.P. for deep penetration. The smaller bomb was the 12,000lb Tallboy, while the larger was the 22,000lb Grand Slam. The Tallboy could be carried by Lancasters capable of carrying the 12,000lb HC, but those that carried the Grand Slam had to be specially modified. Their bomb bay doors were removed, as were most of the defensive armament. This allowed them to carry the heavy bombs up to the altitude required for proper operation.

/u/thefourthmaninaboat has given a splendid run-down of the heaviest bombs dropped by the RAF. At the start of the war the medium bombers of Bomber Command carried a bombload of between 1,000lb and 4,000lb, typically comprising 250lb and 500lb General Purpose (GP) bombs with thick cases (less than 25% of the weight was explosive). As the four-engine Stirling, Halifax and Lancaster came into service with a bombload of up to 14,000lb the GP bombs were supplanted by Medium Capacity (MC) bombs with 40-50% of the weight being explosives, and larger High Capacity (HC) 'blockbusters' or 'cookies' with thin cases allowing 70-88% of the weight to be explosives. Nina Burls gives a good rundown of the developments over 1939-45 in [Journal 45] (http://www.rafmuseum.org.uk/documents/Research/RAF-Historical-Society-Journals/Journal_45_Seminar_conventional_weapons.pdf) of the Royal Air Force Historical Society, including a breakdown of the 955,044 tons dropped by Bomber Command as follows:

Fragmentation (F)	qty
20 lb	5,000

High Capacity (HC)	qty
2,000 lb	28,633
4,000 lb	68,000
8,000 lb	1,088
12,000 lb	193

Medium Capacity (MC)	qty
500 lb	403,000
1,000 lb	253,800
4,000 lb	21,000
12,000 lb	854
22,000 lb	41

General Purpose (GP)	qty
40 lb	49,939
250 lb	149,656
500 lb	531,334
1,000 lb	82,164
1,900 lb	2,141
4,000 lb	217

Incendiaries	qty
4 lb	80,000,000
25 lb	20,000
30 lb (phosphorous)	3,000,000
30 lb ‘J’	413,000

Semi-Armour Piercing (SAP)	qty
500 lb	11,600
250 lb	7,000

Armour-Piercing (AP)	qty
2,000 lb	<10,000