after all at the design stage it would not be that hard to add in a pair of 21161s so some EQ.
It is when the standard 21161 software doesn't have room eq in it... or supports two 21161N's!

The ARCAM's are based around the Crystal solutions that are dedicated audio DSP's with code supplied by the chip manufacturer that exposes a control interface. ARCAM do the integration bit of configuring the DSP's via this interface, but if the software doesn't support a feature (like Room EQ) then it would be rather tricky for ARCAM to add it.
Most AV manufacturers are in the same position as it's quite expensive to do custom DSP development as you need specialist software staff and all the kit that goes with it. The only UK manufacturer that I know of that does it's own DSP software development is Meridian.
When we did the AV32DP and AV192R at TAG we went half way and had Analog Devices do the core decoders and licencable technologies. We then ported this to our twin DSP platform and then designed and coded the rest, hence giving us a fair bit of control of the feature set and algorithm quality in the product.