One day I will stop apologising for all these questions but as I am British I will start with another 'sorry for a silly question'

Is there any way of finding out/ calculating the taxes and fees likely to be payable on a fully redeemed J class flight? For example (and all availability issues aside) can I search anywhere for the fees due on say a flight from LGW to the Caribbean or from LHR to South Africa? I don't have the Avios available at the moment to do a dummy booking.

Thanks.