Did you enable "Provide search suggestions" in the Search Settings panel?

Yes, its enabled

Check the corresponding pref in about:config:

1. Type about:config in the address bar and enter
2. Accept the warning
3. Find the line browser.search.suggest.enabled
4. If the value is false, double-click on it to change for true
5. Close the about:config tab

It's already set to true

Does it happen in Safe Mode?

1. Click on the menu button in the top right-hand corner
2. Click on ? below and select Troubleshooting Information
3. Click on Restart with Add-ons Disabled

You can also check the History settings.

To see all History and Cookie settings, choose:

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• [x] remember search and form history

• https://support.mozilla.org/kb/Options+window+-+Privacy+panel

Where did you get that Google UK Search plugin? The standard google.com search plugin has an extra URL in the file specifically for fetching suggestions, but perhaps that line is missing from your Google UK search plugin and needs to be added.

Or rather than edit it by hand, perhaps experiment with one or more of the Google UK search plugins available on this page to see whether they work for you:

http://mycroftproject.com/google-search-plugins.html

I tested the one that is for results from the U.K. and it shows suggestions for me.