Just get EPA/DHA or fish oil/krill oil. It's better used by the body. ALA supplements (flaxseed) are mostly for people who have vegan/vegetarian moralistic beliefs. The conversion is limited to EPA/DHA.
The wikipedia article on phytoestrogens details all the foods you eat everyday that are considered healthy that no one mentions have phytoestrogens. Phytoestrogens are a different compound than estrogen. http://en.wikipedia.org/wiki/Phytoestrogens
(scroll to Food Sources and Health Risks and Benefits)
--If you have a problem with me citing wikipedia, a secondary source, you are welcome to click the footnotes which lead to the primary Pubmed articles referenced, which I checked prior to recommendation.
To say that consuming soy and phytoestrogen containing foods is still controversial is siding with anti-soy political activists. The truth is, if people figured out how great soy is, the cattle/dairy farmers would lose business, greenhouse gases would decrease, and more people would be not malnourished. It's a cheap complete protein (soy), and there is nothing wrong with phytoestrogens.
Anecdotally, I have been consuming upwards of 70-80g of soy protein per day since I was 16. I had my testosterone checked at age 20 for other reasons and it was 835 ng/dL, which at that laboratory, was considered 10 points too high (but they did not find it cause for alarm). 25g per day is the minimum required for the health benefit (according to the labels). I have had no side effects and if anything, am leaner than meat eaters due to the lack of fat and increased fiber content that comes with using crushed soybeans as your protein source. Current research shows it reduces the risk of prostate cancer in men. It's even safe for people who have had breast cancer.
What do they put in animals to make them have meat, by comparison? Real hormones. No one complains about that. Even if phytoestrogens have an effect, it's not like the body wouldn't adapt by downregulating the receptors that bind the phytoestrogens to regulate downstream effects of consumption, provided excessive increases in consumption (not maintaining a consistent intake) do not alter the expression.
LAST AND NOT LEAST, your personal trainer is stepping outside of his scope of practice to recommend flaxseed to you, which would be in the nutritional counseling domain, unless he has formal education in nutrition (at least a BS, preferably the registered dietitian (RD) credential, both which require 4-5 years intensive science education--all other nutrition credentials out there are bullshit weekend courses). Heck, does your personal trainer even have a BS in exercise science, kinesiology, exercise physiology? FYI the industry is backwards, where they require a certification instead of a degree to be hired at most gyms, and that's because certification bodies pay gyms to hire their certified trainers...
--Edit: above paragraph applies to the US, not sure about the UK.