I don't think they'd have learned much from the visuals of a balloon (it takes marginally higher resolution images than a satellite but so do lots of less obvious stuff like aircraft and consumer drones that are perfectly entitled to be in US airspace). But the US was pushing the UK not to include Chinese equipment (and indulging in some very loud anti-China rhetoric mostly for domestic consumption) under the previous administration but was too cautious to pull the trigger, so them not being too cautious now is new information. Unmanned balloons are obviously a lower risk way of testing that than other ways it could test the US tolerance, and may play well with Chinese domestic audiences too.

> I don't think they'd have learned much from the visuals of a balloon

I suggest a balloon payload would be better aimed at monitoring (and establishing patterns of) RF traffic rather than visual data.