Sure! Here are the rewritten sentences:

17. Apples are generally less expensive than oranges.
=> Oranges are not as cheap as apples.

18. I haven't visited Bristol for three years.
=> The last time I visited Bristol was three years ago.

19. If he doesn't call immediately, he will not receive any information.
=> Unless he calls immediately, he won’t get any information.

20. This is the first time I've met such a famous person.
=> I've never encountered such a famous person before.

21. I last saw him during my student days.
=> I haven't seen him since I was a student.

22. The journey from London to Edinburgh takes six hours.
=> It takes six hours to drive from London to Edinburgh.

23. Unless it rains soon, millions of pounds' worth of crops will be lost.
=> If it doesn't rain soon, many crops worth millions of pounds will be lost.

24. He is more persuasive than his brother.
=> His brother does not speak as persuasively as he does.

25. "I no longer love you," said Eric to his girlfriend.
=> Eric told his girlfriend that he didn’t love her anymore.

26. Although I admire his courage, I think he is foolish.
=> Even though I respect his bravery, I believe he is unwise.