Is it true that the English more or less forgot about their Roman heritage for several centuries? If so when and how did they rediscover it? Also when did it become acceptable for European artists to create works inspired by Roman and Greek mythology, rather than just Christianity?
Britain more or less completely fell apart after the Romans withdrew. Even before the Anglo Saxon invasions, it was really unstable. With no educated aristocracy to take care of the infrastructure, cities fell into ruin and were abandoned, they eventually stopped using money and regressed into bartering. So already there was a shift going on in their culture, then the Anglos, Saxons and Jutes came and definitively Germanized the island.
Here's a nice podcast that covers the subject pretty well:
http://www.hi101.ca/?p=294
As for the last two questions, the Renaissance.
>>3206352
There is little to no proof the Roman Empire even existed. Everything we know from the Romans and Greeks has been rewritten by Jews and Arabs to their taste.