The expression for $\%$ change in exchange rate for USD$/$GBP is given by

$[$S$($t$,$ USD$/$GBP$)-$S$($t$+1,$ USD$/$GBP$)]/$S$($t$,$ USD$/$GBP$)$

$[$S$($t$+1$USD$/$GBP$)-$S$($t$,$ USD$/$GBP$)]/$S$($t$+1$USD$/$GBP$)$

$[$S$($t$+1$USD$/$GBP$)-$S$($t$,$ USD$/$GBP$)]/$S$($t$,$ USD$/$GBP$)$

$[$S$($t$,$ USD$/$GBP$)-$S$($t$+1,$ USD$/$GBP$)]/$S$($t$+1,$ USD$/$GBP$)$