It is the NRL1 form. Means they pay you gross. It DOES NOT mean you are exempt from tax in the UK as shown above.
It doesn't matter that you don't live in the UK. These are the guidance notes:
http://www.hmrc.gov.uk/cnr/nrl1_notes_bw.pdfPurpose of form NRL1
United Kingdom (UK) rental income is liable to UK tax
whether the landlord lives in the UK or not, and
normally a letting agent or tenant has to deduct tax
when paying rent to any landlord whose usual place of
abode is outside the UK. Form NRL1 enables an
individual whose usual place of abode is outside the UK
to apply to have his or her UK rental income paid
without deduction of UK tax. But our approval of the
application does not mean that the rent is exempt
from UK tax, and we may later ask the individual to let
us know whether they have any tax to pay by
completing a UK Self Assessment Tax Return.