well, tbh, reality, if those are your criteria, it would be incredibly easy for us to claim we are not a couple.
the flat is in dh's name only, and the bills are all registered to him. he has post delivered there, naturally (bills etc, inc credit cards, as it is easier to keep track of what is registered where if all at one address).
what he puts into the household pot here (as in monies "paid" to me I am a sahm) is less than he owuld end up paying in maintenance payments if we were separated.
so, a GP registration (he is not registered where we currently live either, btw - we rarely see docs, so is probably registered still at our last address anyway) would fulfill your criteria, or it owuld be easy enough to make out that it did (ie him paying maintenance)
but the fact is of course, despite that we are living together.
and in the case you highlighted, someone spending 6 nights a week, paying ofr some of the household stuff (ie money towards food etc) and doing household maintenance tasks, as well as socialising a lot - yep, in my book that's as good as living together. whether they would want it to be called that for tax credit purposes is another thing, but the fact remains that they are living as a couple.