Multiple questions about Route 53

Some of the things of your list are not DNS related, so Route 53 won't help but you might find something in other AWS services, below my quick comments:

Full registrar services?

Here is the list of Domains you can use Route 53 as a registrar: http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/registrar-tld-list.html

UK specific DNS suffixes?

Search on the page above.

Unlimited DNS queries?

There isn't a limit, however you pay per query.  (customer may be asking for an unlimited plan)

2 factor-authentication?

The access to  the Route 53 console (for DNS administration) is controlled by IAM and therefore you can enable 2 factor authentication. http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_mfa.html

Sub-domain delegation?

Yes.

Single sign on?

Yes, again the access to  the Route 53 console (for DNS administration) is controlled by IAM and thus you can implement SSO: http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_providers_saml_3rd-party.html

Sell ssl certificates?

Route 53 don't, we have another service for that , it's called AWS Certificate Manager, however we don't sell certificates, we give them for FREE :)  https://aws.amazon.com/certificate-manager/