The RAF started off the war believing in daylight bombing, but some nasty early experiences in unescorted raids soon showed that this was not a good idea (RAF bombers at that time had little armour and only a few .303" guns for defence - they got chewed up).
Bomber Command therefore switched to night bombing and concentrated on this thereafter. At first, the accuracy was terrible because the RAF had never developed accurate blind navigation aids, but as the war went on technology and techniques improved and by the end of 1943 a night bombing raid could be as accurate as a daytime one.
Day bombing by the USAAF was generally accurate - in good weather - but in bad weather they relied on ground-mapping radar so their accuracy was no better than Bomber Command achieved in bad weather at night.
Ironically, by the end of the war night bombing had become more dangerous than day bombing. The USAAF's day bombers were not only well-protected and armed, they crucially had excellent escort fighters with them all the way (of course, it was a different story earlier on when the USAAF tried unescorted raids). At night, the growing skills of the German night-fighter defences meant that the Lancaster bombers lost an average of one plane in twenty, every mission.
The RAF carried on bombing even with heavy losses because it was a way of demonstrating to the Russians that we were still fighting with them. There was no other way to take the war to Germany until after D-day.
Late in the war, it was really too late for Bomber Command to switch to daylight bombing. The bombers weren't suitable for it (except at the end, when there were few German fighters about) and the RAF hadn't developed any long range escort fighters (no need for them at night) and anyway, as has been said, there were advantages in being able to maintain 24-hour attacks. The RAF and USAAF often coordinated their attacks to keep hammering a target.
Tony Williams
New book: Rapid Fire - The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces.
Details on my military gun and ammunition website:
http://www.users.globalnet.co.uk/~autogun/