If we had (heaven forbid) say a loco and could just build plenty of cheap unpowered coaches ,
Even 'unpowered coaches' aren't cheap any more - the only thing they lose compared to an MU vehicle is the power equipment, the cost of which ends up in the loco instead. Railway passenger vehicles are expensive to build partly because the labour content is high (think about all those interior fittings and furnishings that have to be hand installed due to space and access limitations).
and that's the fault of the industry over past 25 years focusing on fun sized all singing all dancing trains when simple, and comfortable would have been better.
But apart from the IETs (and FLIRTs) most of what's been built are simple EMUs and DMUs as replacements for old BR-era trains and for capacity upgrades. There's been over 2700 Electrostar and over 2000 Desiro & Desiro City EMU carriages alone built since privatisation. Those are not 'all singing and dancing' trains, just the modern equivalents of the standard EMUs of old. Adding the post-privatisation DMUs and more recent Aventra EMUs (around 2600 carriages) and CAF trains would add considerably to the total of 'simple' passenger vehicles.
That total completely dwarfs the number of 'all singing and dancing' IET carriages built to date, for example (according to the Hitachi UK website, there are 182 IETs 'in service', which if you assume an average length of 7 vehicles per train that's around 1300 carriages).
So no, the industry hasn't been focused on 'fun sized all singing all dancing trains' for the past 25 years - it's mostly been ordering boring basic stuff.