If there’s a fee involved, you would in the first instance account for the PayPal fee like you would if it was a GBP transaction - split the fee from the total to get the transferred amount. There’s more on that here.
Either way, when you tag the actual balance that was given to you, it will prompt you for a GBP equivalent. I’ll use the same figures as your example above as an example:
This would be the same going the other way too (from GBP to USD).