I Just Drive,
the phraseology you refer to is because glideslope signals in the UK are only guaranteed protected to just beyond 10 miles as opposed to the localiser signal which is guaranteed to somewhere around 20.
As airports are becoming increasingly busy then you are more likely to be establishing on the localiser at distances greater than the protected range of the glideslope. The controller is therefore using phraseology such as " Descend to altitude 3,000ft then further with the glideslope" to ensure that you capture the glideslope from below inside the guaranteed protected range.
Baggy.