Screwfix have a number of offers including a Bosch "GBH-2-24 DSR" 2kg
SDS. Oddly it weighs 2.4kg, so should I assume that the "2kg" relates to
something other than the weight of the drill itself?

It just refers to the approximate weight of the machine. They come in 2kg,
4kg, 6kg etc. 2kg are the most flexible.

Also, it is sometimes used erroneously in place of "joules" for impact
energy, which is often similar to the weight in kg. I don't think Screwfix
makes this mistake, though.

Christian.