In my experience, the HHonors website sometimes has erroneous information as to whether non-smoking rooms are available. On top of that, we're talking about a stay that is MORE THAN EIGHT MONTHS AWAY. Many, many things (cancellations being not the least of them) will happen between now and then.
I would call the HHonors Diamond desk first. Paying for a long-distance overseas call to London would only be a last resort.