London was bombed heavily and a lot of the city was destroyed. However, I have never heard of any important buildings or artefacts being destroyed. Did bombings destroy any important buildings, art pieces, historical sites? If so where the rebuilt?
You can see a list of iconic buildings that were totally destroyed by bombing on this page sort by 'date of destruction' and scroll down to the 1941-1948 section. Note that this is by no means an exhaustive list, there were at least 7 churches designed by Christopher Wren that were destroyed, these are just the most famous examples.
Other major landmarks such as Buckingham Palace, the Guildhall and St Paul's Cathedral suffered damage but survived.
I think the most notable buildings totally destroyed were Christ Church Greyfriars which had been rebuilt by Christopher Wren after the first Great Fire of London (the worst night of bombing is colloquially known as 'the Second Great Fire of London), the Carlton club building which was central to the history of the Tory Party and the Great Synagogue of London which was the focal point of Jewish life in the England since the late 18th c.
Mostly buildings were rebuilt, the common exceptions being churches whose ruins were kept untouched as memorials to the dead. You can still see these shells of former churches in many British towns and cities.
The City Temple URC, a church in which I lived, and raised within the Corporation (Holborn Circus) was demolished, leaving only the facade and one rear corner.
It is known fit being launched by Cromwell's fellowship, and is on the protected list.