Sorry Bert, it's not clear to me - is the problem with downloading the gpx from CT or with uploading it to Strava?

Assuming you are using the CT website if the problem is in downloading from CT have a read of this: https://cycle.travel/advice/map/gps

Transferring a route manually

You can also download a ‘GPX track’ file, which can be read by all brands of GPS and many apps. This will show the route as a line on your GPS screen. Click Download GPX track, and the file will be downloaded to your computer or phone. 

Also:

Different types of GPS file

There are lots of different formats of GPS file. You can access these by clicking More download options. GPX tracks are simplest, but you can also choose a ‘TCX course’ which includes turn-by-turn prompts. cycle.travel offers these formats:

• GPX track: This is just the line of your route – no instructions. Practically everything will understand this.
• GPX route: This is an icon for each turn on your route (but no line). It’s good for GPS devices without much memory.
• TCX course: This has both the line and the turns. It’s best for Garmin Edge and Wahoo units.
• TCX activity: This is like a TCX course and is easier to import into Garmin’s Connect software.

When it comes to uploading to Strava, have a read here: https://support.strava.com/hc/en-us/articles/206811950-Uploading-Route-Files#:~:text=You%20can%20create%20a%20Strava,use%20to%20create%20a%20route.

I believe Strava will also accept TCX files. 

It may be that trying the different forms of gpx file (or TCX) will solve your problem. It may also be an issue that there is a difference between the process on the Strava App and the Strava website.