There are two types of JSA.
Assuming that your National Insurance contributions are up to date, the first 6 months of JSA is 'contribution-based'. That type of JSA isn't means-tested but it doesn't qualify for other benefits, help with mortgage interest payments.
Contribution-based JSA is paid for up to 6 months. After that you cease to be eligible for it but you can then be considered for income-based JSA. That type of JSA IS means-tested so if, for example, your partner is in employment you might not be eligible for it.
However, if you can get income-based JSA you then automatically become eligible for other benefits, such as free NHS subscriptions and (most importantly in respect of your question) SMI (Support for Mortgage Interest). It should be noted though that SMI is only payable for a maximum of two years:
http://www.direct.gov..._low_income/DG_180321
Chris