Originally Posted by
Dan72
It appears that the exemption from DATV is only for journeys to or from a qualifying country (eg US) for which you hold a visa for (or residency). It's not a standard exemption from DATV to transit the UK to any third country, eg Portugal. The SIN may have been right to refuse.
I think it may be worth reading
both sets of Timatic information posted above to see why this restriction doesn't apply in this case.