First of all, you'd have to pay a significantly higher APD to fly in WTP. For some people that's a deal-breaker right there.

I believe surcharges are the same in both cabins on this route, so no added cost there, were you to use Avios to upgrade.

Perhaps you should just keep an eye on any special offers to upgrade. You'll see them when you Manage Your Booking.

You will enjoy the extra legroom and seat width in WTP. How much that is "worth" is up to you and BA...