For a huge program such as Tricare, which covers all US military active and retired -- with coverage near identical to Medicare, except its coverage is valid overseas -- BoI is clever enough to know it covers all the medical coverage stipulations required for an LTR visa. Thus, they accepted the letter I got from Tricare, even 'tho no coverage numbers were presented. I would assume, if your US insurer is a known entity, a letter from them, like my Tricare letter, might do the trick. Or maybe not, if, unlike Tricare, their coverage is not 'one size fits all.' In which case, yes, some coverage numbers would probably need to be provided.