Not sure what your question is. British Airways charges for advance seat selection unless you have high status in their mileage program or are flying First Class. For Business, we paid $278 for two seats (window and aisle) on the upper deck of a 747 for our next flight. I think that is the equivalent of 100 GBP. Lower deck seats are a bit less.
If there are no seats available for you to choose on the outbound flight, they must be holding some back for premium status customers. Or maybe there are no two seats together left, meaning others booked them all before you got a chance. For the return, $300 is more than we paid for our 2 seats, but I paid that last August and perhaps the fees have gone up.
In any case, it is standard BA policy. You can choose to wait until check-in, 24 hours in advance of the flight, and select seats fro free at that time. Or you can pay to have the certainty of sitting together in seats you have chosen for yourselves.
If you can check in exactly 24 hours in advance of your flight, you can view the seats that have been assigned to you, and might have the opportunity to change to ones you like better.