The heroin you get in the usa isn't the same H that most of Europe gets. US H is #4.
Afghani H is almost always #3 which is what you'll get in the UK. You can't inject it without adding a acid but it can be smoked on foil unlike #4. ( usa also has black tar H, different process.

In UK you can make big Big money dealing H even as a small timer. But be prepared for trouble from cops and robbers.

Don't think fentanyl is really in the UK much.