[AaronLancs]

If ITV buy all3media, that probably accounts for half the indie TV production in the UK...

Well you have already a few noted 'labels' / 'imprints' within All3Media. A few of the more notable for a UK audience are:

Bentley Productions
Known for drama productions especially the internationally renowned Midsomer Murders.

Lime Productions
The production outfit that was originally known as Mersey Television. Hollyoaks is their main long runner.

North One
Mainly documentary and features driven with The Gadget Show among the various productions made.

Objective Media Group
Founded by Andrew O'Connor and best known for most of Derren Brown's output.

Studio Lambert
Need I say more.

So I would safely say that it indeed is a considerable chunk of the production market.