I believe there are two separate aspects in play here.
I'm referring to one - whether the IPv6 Prefix Delegation assigned by HO to their customers is fully routable or not.
Other posts are referring to another - whether there may be a firewall policy of some sort in play.
Regards the routable aspect, the IPv6 PD assigned to customers by HO (at least, to me) is fully routable. There is no impediment put in place by HO at the IPv6 routing layer
to impede access to your IPv6 allocation from the Internet. So, if you are using your own router and want to access your IPv6 PD from the Internet, you should be able to do so.
A seperate issue is that there may be a firewall policy on whatever router you are using that blocks access to your IPv6 PD from the Internet. I don't use the HO router, but other posts here suggest they may well block this by default. I would argue that this is the correct and sensible option for a consumer oriented service. If you don't want that, you can always use your own router instead
I hope that's a bit clearer.