So, how much is it worth to you to pay to choose your seat? Try using a travel agent as they seem to have an ability to do this, over and above what airlines offer on their websites, but the overall ticket cost may work out more expensive.
The BA lark of charging for a seat is a bit of a joke IMHO, but it's the way that the world is going. I flew LHR-YVR and back earlier this month - I would have had to pay a lot extra to choose a seat so declined to pay up to £65 each way for the seat that I chose for free anyway - I only booked about 7 or 8 weeks before I flew and by then, it appears that all, or almost all, travellers in the cabin I was in (premium economy), had already paid to choose the prime window and aisle seats (or were high enough in the Airmiles club to be able to choose their seats for free at time of booking). I am almost always travelling on a strict budget and don't have the will or funds to blow a significant sum on choosing my seat having paid extra for PE over cattle class - I'd prefer to use money on that upgrade rather than choosing a (window) seat in cattle class.
Outward bound to YVR I had an aisle seat but in the centre of the plane, with the only empty seat in the cabin next to me, for a change!
Inward I had a "middle" seat in the centre of the plane but as I did my best to sleep for the entire flight, the lack of easy access to an aisle wasn't an issue.
(Bigger issue for me is that when I booked, BA were using the A380 to YVR but from October they switched to a 747, which is far noisier and IMHO Not as comfortable; I was really looking forward to the A380 again and felt very short-changed having to go by 747)