Just do a normal reward search on ba.com, select the BA premium voucher and the specific voucher number, and search on MAD to HAV (don't worry HAV doesn't auto complete), and select passengers to 1. Should work as normal. For that route I get a return in J at:
42500 Avios + £ 221.70
36150 Avios + £ 371.70
27650 Avios + £ 541.70
21250 Avios + £ 711.70
14900 Avios + £ 881.70
8500 Avios + £ 1,051.70
Note going to Cuba may preclude you for using an ESTA to go to the US btw
https://www.flyertalk.com/forum/brit...lled-cuba.html