First thing, the car looks great. I'm not Welsh but I like the paint job, it harks back to "racing stripes" and is a subtle way of getting a personal statement on the car. Great.
Right, handling. I'm far from an expert in this field and there are plenty more qualified than I to comment, but heck, that's never stopped me before so here goes....
1. As others have said, geometry is key to these relatively light cars. Any toe-out at the rear is very bad news on mid engined cars in general and the rear toe in specified for the TC Europas is quite high, you can go to something like 1/4" and still be in spec. The other thing about the rear geometry is getting it aligned to the car axis so the car won't crab sideways. For a rear handling issue that would be my first port of call.
2. As BDA said, the rear camber looks fine stationary but quite a lot of negative on the unladen wheel when cornering on that first left hander. There's a lot of movement in the camber on these cars (I geekily measured mine
) but it generally goes positive when the wheel is unloaded, not negative. Very difficult to tell from a video though, you can only get a sensible answer from your geo check.
3. You mention standard dampers/springs with different wheels. A couple of things on this, firstly about tyre pressures if you've changed tyre width or profile because modern cars tend to have higher pressures with lower profiles. I'm running my 50 series tyres relatively soft by modern standards, 18-20 front, 28-30 rear, might be worth checking yours if you've just come from the tyre shop.
Secondly the car looks very low at the front going on the wheel arch clearances, but again that's difficult to tell from video. If you have a significantly lower front than rear it would make me think about relative roll centers, if the rear is much higher then you might be inducing rearwards instability. No idea on numbers, that's pure guessing. With the body/chassis changes on the TC they often look nose high compared with the S1/S2 and it's difficult to get the same look on the TC using OEM parts.
4. You mentioned UJs and I'll assume that when doing that job you'll have seen the lower link bushes are in good order, not bent, etc. The other place I'd look is the large rubber bush where the trailing arm locates in the chassis and where you do the alignment. There's a lot of rubber in that bush and if it's perished/cracked/split then you could be inducing alignment changes under braking/acceleration or I suppose even cornering. Worth a check ?
Finally, looking at the speed on the road I don't think this is your problem but I notice there's no front spoiler on the car. The TC had one fitted to improve high speed stability (rather than a marketing exercise) so if you find the car wanders at high speed I'd look there first.
Brian