Ответ:

1. Joan went on her tour of London after she had looked at the map carefully.

2. The children bought big ice-creams after they had climbed to the top of St. Paul's.

3. The Tower became a popular tourist attraction after it had been a prison.

4. After the group of tourists had come back from a boat trip, they took a taxi back to their hotel.

5. After David had worked in a factory for 3 years, he bought his taxi.

6. David hadn't had a job for a long time when he decided to move to London.