If you mean signing certs as a self-employed spark, and that includes "foreigners", make sure you have suitable insurance, unless you are extremely wealthy or extremely poor. Get it wrong and you'll end up the latter pretty quickly.
A suitable testing qualification wouldn't go amiss. A quick perusal of the C&G chief examiner's reports shows that the pass rate is woefully low (most sittings less than 40%), so in my opinion it is a qualification that will go a long way to demonstrating your competence.
And you'll also need a calibrated tester of course. Not forgetting your Part P registration for notifiable works.