The biggest risk is from a data breach and this information being accessed by unauthorized parties, but that is something all online services are at risk from. The absolute worst way to implement this will be to contract it out to a third party. If it is built and maintained by civil servant developers who have already proved their mettle with a variety of govuk services then I would have confidence in it. If it is farmed out to Fujitsu or some other 3rd party then it will be an shithshow and an expensive one at that.
The potential for BritCard to be used for surveillance outweighs the benefits of convenience tenfold... Privacy is not something we should compromise for easier access to services- what starts as a way to "streamline services" can quickly turn into a horrible mechanism for tracking citizens under the guise of security..
For reference:
https://www.gov.uk/government/news/new-strategic-partnership...