Yes White Lane is on the HHH. Last year it was right at the start (leaving Bix) - with a Sonning Common start it must be a few K in.
It doesn't bother me much - it's so bumpy I crawl down there anyway. Frost could be a problem in Jan, but not this week <crosses fingers>. It's a pretty horrible road, but a sacrifice worth making to avoid riding thru Henley Central.
(TBH if the event started 2 hours earlier I'd 'long-cut' thru Henley, but it will be busy when we get there, I expect.)