check the AHRC website for details, but in general, if
- you have EU citizenship
- you have lived in the UK for the last three years before your PhD starts
-then you are eligible for full studentships (ca. 12'000£/year plus fees, for three years)
if
- you have EU citizenship
- you have lived in the EU (outside of the UK)or Switzerland for the last three years
-then you are eligible for fees only studentships.
be aware however that these are (both) very competitive.
some universities provide their own studentships which are sometimes independent of citizenship and previous residency.