I have started adding postcodes for proprieties which I have got from friends/family so I know that the data is free from copyright.
This site https://postcodes.io/ claims to be open source. Would that mean that the information I gather from here be usable on OSM?
If I choose to verify postcodes does this process also have to be from open data also? Possibly https://getaddress.io/
This is a UK specific issue, so probably best discussed on the talk-GB mailing list.
If they have legitimate data, they should create an OSM account and enter it directly.
Anything else would require the full bulk imports procedures.
“Open source” is not enough. You need to establish the exact terms of their licence, whether OSM are complying with their licence, and whether you actually believe that they have obtained the data without violating database rights.
This site is being run as a profit making business, and I would say, to the extent that they could legally enforce it, they would not permit it to be used to populate another database. If they couldn’t enforce it, you should expect to be barred as a user. They prohibit the re-use of their intellectual property, but don’t say what that property is. However, they have almost certainly created their own database rights by integrating the data from other sources.
For OSM, I would suggest looking at the upstream sources they use. However, I imagine most of those have already been considered by OSM, and either are already being used, or have been rejected as legally unsafe. In particular, I think the Land Registry has been considered legally unsafe, because it is believed that PAF data (or maybe National Gazeteer data) is used in creating their entries. If so, that would rule out the property sales information they are using.
I would suggest discussing the matter on the UK mailing list https://lists.openstreetmap.org/listinfo/talk-gb