Do this all the time when flying to visit inlaws in MEX. Since SO holds a Mexican passport the UK tax is not applicable (even when traveling on US passport). After much trial and error what works best is to book and complete purchase, then call and ask for UK tax to be removed. You may need to speak with tech support, but I have had some Elite desk agents do it, too. The amount of the UK tax is refunded to your OFP.