You are correct that only a small amount of ALA gets converted to DHA.
I don't think that is a reason not to use any flaxseed/flaxseed oil as there are other benefits and it will be omega 3 DHA sparing, in that some of the uses that DHA may have been used for may be achieved by the ALA or EPA..
However you are still lacking DHA and if you won't take a fish oil derivative then maybe you would consider an alagal version
Bluebonnet Nutrition, Early Promise Prenatal, Gentle DHA 200 mg, 60 Veggie Softgels DHA (docosahexaenoic acid of algal oil) 200 mg per capsule 100% Vegetarian $26.36
But you can soon see how using fish oil you double quantity and pay half the price.
Omega 3 / DHA Fish Oil Country Life, Gluten Free, Ultra Omegas DHA/EPA, 120 Softgels $13.26
The examples come from IHERB because they offer cheapest shipping to UK. They also offer introductory initial discounts if you look online for some peculiar reason we are not allowed to share these here although helping others in times of economic austerity seems totally in line with site policy to me. However searching for IHERB DISCOUNT CODES will find plenty and if you add my name you may find mine.