Can I apply from outside the uk?

You must be able to prove it that you have the right to work in the UK to apply for a HMRC job

If you look at the job description on Civil Service Jobs (towards the end of the page), you will find the job requirements (e.g. needing to have lived in UK for 5 years to pass security checks, right to work requirements, citizenship requirements etc.) to be able to apply.  If you are looking for a British civil service job overseas, the FCDO has a separate website for those roles outside of the UK that are open to those not living in the UK but will have local job requirements.