So I tried to do a dummy booking, but when I went to select seats the economy seating chart didn't come up. I called the helpful TAP web support who told me I could pay for exit row from any fare class, including discount, and if it wasn't showing at the booking stage I could select and pay from Manage my Booking.

So I purchased Discount and then did just that. Seemed to work fine. I didn't see the need to spend another £40 for things I didn't need.