In country X, an indirect quote is foreign currency over domestic currency, or FC/DC_X.  The quote 0.55500_$/GBP is FC/DC (or indirect) to an investor in Great Britain and DC/FC (or direct) to an investor in the United States.

The bid-ask spread in European terms is foreign currency per dollar (FC/$) so you are being asked to convert the bid?ask spread to GBP/$. To convert to GBP/$ just take the reciprocal of each number and reverse the bid?ask spread.

1 / ($1.8709/GBP) = 0.5345 GBP/$ and 1 / ($1.8841/GBP) = 0.53076 GBP/$. The bid?ask spread is now 0.5308 ? 0.5345.