I think there is a good reason why impression based sketch shows are generally not that good.
Many great comedians have started their careers as immpressionists, but then moved on to their own creations, eg: Steve Coogan, Rob Brydon, Harry Enfield, Peter Serafinowicz etc. They may still use this skill in their comedy (The Trip, The Peter Serafinowicz Show), but it is not the focus of their work.
The ones that don't have the writing/acting skills to move on from doing impressions have no choice but to stick with it, eg: Jon Culshaw, Alistair McGowan, Debra Stephenson etc. So the end result (particularly the writing) is not of the highest quality.
There are exceptions of course, possibly Rory Bremner. Also, Stella Street was very funny.