New to Garmin. Apple health not sharing my history with Garmin Connect.

You cannot sync data from Apple to Garmin Connect. Only the other way around works.

Thanks for your reply - just what I expected from Apple. 
just confused why I had that prompt during set up! It mentioned the name of the software that I was allowing it to access the information with- I wish I’d have took note of its name. 

You can check if you given permission to any app to send data to Garmin Connect in this page:
https://connect.garmin.com/modern/settings/accountInformation 

Zwift is one of the few services that can send activities to Garmin Connect and it looks like this:

Note that is says "to Garmin Connect" and the switch is for "Activities".

Martin, it is 100% Garmin's fault.  Apple makes every single bit of your health and fitness data available to any app you give permission to.   The only data that Garmin connect requests access to is Date of Birth, Height, Sex, and Weight.   There is nothing stopping them from releasing a version of Connect Mobile tomorrow that also pulls in your complete workout history, all health data, etc. from HealthKit, which includes the Fitness and Health Apps.  I agree that Apple has a reputation for being a walled garden, but this is one time it is the exact opposite case.    Please be careful about pushing data from Connect to HealthKit as Garmin has done a horrible job of implimenting the API that direction as well.   They are aware of all these issues (have been for years) and have no plans to fix them per their representative here.  You can search this forum for a number of threads about this.  

I'd love to see the prompt if you can get it again with a screen shot.  Since Connect doesn't even try to grab that data I'm really curious.   I do know that Connect prompts you to give write access to a ton of things.   Most of which it either won't use, or will use incorrectly.  Is there any chance they were asking about writing your fitness history the other way?

e7andy, that's the setting for using Garmin's web based API.  (which is how strava, zwift, mfp, etc. connect to your Gamin data).   The Apple data model is different.  Because they have decided to only store your data on your phone so that it is encrypted and only stored locally (a controversial decision) you have to control access to the data on your iPhone in the settings app.  It' s under settings - health - data access and devices.   And unlike the Garmin API it is extremely granular.   (for instance you could give an app just the ability to see your diastolic BP, but not your systolic if you wanted.).  The list under each app will show what it has asked to be able to read and write (both separate settings) by item.   That list is the most that an app can do/have since it's all they have asked for.  Each value has a switch you can set to allow that permission.       If Martin checks his settings, he will see that Garmin hasn't even asked for anything related to workout history.  (but has asked for weight for some weird reason since they don't seem to do anything with it).  

No it definitely was suggested that it was going to populate Connect with your previous fitness data from the iPhone. Either way though I did give it permission and it didn’t work  thanks for your response. 
I’ve just realised that it looks like the Apple fitness hasn’t been receiving any data and is empty on my first full day with the Garmin. Looks like you were right - the Garmin’s data isn’t working back to the iPhone either. 

Thanks for that info. In settings though it says it reads & writes all the data!! Wtf??! See images

Apple fitness seems to have the data but not showing it in the rings?

Right underneath where it says "Data from "Connect"" is where it will show what it is trying/able to read, as it is managed separately from write.  Everything you have shown there is under the "write" section.  So Garmin can tell Apple, but won't listen.  

It's not showing it in the rings because Garmin ignores the API standard.  They don't time stamp anything the way they are supposed to so, even though you went for a walk, Garmin tells apple you did it in zero seconds.    Connect is also well known to write double data for calories, and then sometimes either remove them later or apparently add even more the next day.  Basically the Garmin integration into HealthKit is a mess.  They know it.  They've known it for years, they said on the forum they don't plan to fix it, but yet they have no problem advertising it......