Hi, welcome to the forum!
I don't think your residency in Spain would affect your processing here in Costa Rica, as the law is basically the same no matter if you are US citizens, UK, Spanish, Italian, or Martian. You can have a look at the requirements via these articles:
Overview -
Retirement in Costa Rica]Complete information -
Costa Rica residency from Go DutchThe main obstacle you will face is maybe making sure that your British documents are authenticated, which will have to be done by the Costa Rican embassy or consul in Britain, rather than the one in Spain.
I hope this information helps and don't be shy about asking more question on other topics.