Always a pleasure to see the first ranunculus (Buttercup family) in flower, and it's invariably Lesser Celandine or Ranunculus ficaria. (Although some now put it in its own genus and call it Ficaria bulbifera.)
Here it is flowering on 24th February at the base of the prison wall in Lewes along with the purple dead-nettle and ivy-leaved speedwell.
Gilbert White saw it flowering in Selborne on 21st February, but it usually flowers from March onwards. It is quite common but the flowers only open when it's bright enough. Pilewort is an old name for it because its roots apparently resemble haemorrhoids, for which it was used as a treatment: the good old Doctrine of Signatures.
Greater celandine is totally unrelated. It belongs in the poppy family.