At the start of the period, the risk of heterosexual infection was comparable to being struck by lightning.
An infinitesimal risk.
A couple of hundred out of a population of 60 million could reasonably be described as an infintesimal proportion.
It has risen rapidly but is still a tiny proportion.
Also, in America, this group are not you average heterosexuals.
Their partners are overwhelmingly drug injectors or some other high risk category.
I can not find the breakdown for UK, but why would it be different?