Our courts take 'attempting to pervert the course of justice' extremely serious. Even simply creating a false story to avoid a speeding conviction is likely to result in a prison sentence (of between 6 and 18 months).
So using threats against others, in order to attempt to pervert the course of justice, is going to result in a much longer sentence. Without knowing the exact circumstances, it's hard to give a very accurate estimate of the sentence. As an example, the mother of Sean Mercer, who murdered 11-year-old Rhys Jones in Liverpool was sentenced to 3 years for attempting to pervert the course of justice, but there was no suggestion that she'd threatened anyone. So, I'd guess at a possible sentence of 5 years for the offence you refer to.
Chris