Hi ChrisAlanRobinson,
welcome to the Apple Community
This suggestion may not explain why the call goes to Belgium ... (?)
yet it may help change the situation when you make UK calls.:
Wondering if you may have "dial assist" turned on in Settings > Phone:
and, if so, whether turning it off would help.
Apple Support's article:
Make a call on iPhone – Apple Support
says:
Turn on Dial Assist for international calls: (GSM)
When Dial Assist is turned on,
iPhone automatically adds the correct international or local prefix when you call your contacts and favorites.
___________________________________
You may want to send feedback to Apple:
They will likely not reply,
yet your feedback will reach those who can make changes.
If others are noticing this difficulty too, we may see changes in a future update.
There's a web-form for sending feedback.
Feedback - iPhone - Apple
All the best :-)