Teams could design their own cars back then. A Team Dynamics Integra on the lift, there is some nice stuff on there:
https://www.tegiwaimports.com/blog/?p=3920.
Those rear control arms are very far removed from the simple steel trailing arms of the original car, but they meet the letter of the S2000 rules in terms of pickup points and what not.
BTCC rule makers do not allow this anymore -- however TCR rule makers do, to a point, it's just that the TCR car has to be cheap enough to build to sell at the price cap unless you are willing to make a massive loss (and there's obviously balance-of-performance to reign in any cars that are too fast/too good, so there's no point in doing that).
I wonder why so few BTCC teams are mass-producing TCR cars for sale?! In theory, it should be a lucrative market.
The front subframe looks standard though. It was a pretty crude steel item on DC5s and EP3s (granted they were economy cars so you'd expect that), a far cry from a modern
fabricated aluminium subframe that you find on something like an Alfa Romeo Giulia (it's almost like a small section of a typical aluminium spaceframe Ferrari chassis, very fancy!). The abandoned Golf GTI Mk8 TCR race car (
seen here on a hoist) was intending to run the aluminium front MQB subframe (from the Audi A3) but with some reinforcement braces welded-in.
Should the BTCC, perhaps, seek to emulate the production-based character of TCR?! With standard pickup points, standard subframes and all those things?
Or do fans essentially not care whether or not the control arm pickup points are within 25mm of the standard road car and whether double wishbone has been substituted in place of the (typically) MacPherson strut front and multi-link rear of most hatchbacks respectively?