Is the light being twisted a valid or potentially succesful defence or should I plead guilty?
Well, the offence was committed - I don't think you are denying that. The green light being an arrow imho reduces your chances but I get the point about being green. From the recent photos, that post doesn't look excessively twisted and taken from the right turn lane (and not the view from lane 3 which is the material position?)
As already noted, the time into red means out of court settlements are unlikely. (Will depend on the Force's policy)
On the basis the matter will head towards a prosecution, I guess there's little to lose in pointing out the defect to the Police and seeing if they will consider. (They may not and state it would be a matter for the court only)
...when the 'summons' (SJPN) arrives* then you can decide what to plead. Another option would be pleading guilty but to ask for the court to consider Special Reasons Not to Endorse where all of the following apply:
1. Not amount to a defence
2. Valid reasons that the Court should take into account
3. Not relate to the offender, e.g. you cannot argue you'd lose your job because of it
4. Something that reduces the severity such as a mitigating or extenuating circumstances
Additional court costs could be incurred with the SRNtE so is not risk free.
* they have 6 months to commence the prosecution, annoyingly they often start them close to the cut-off (Just when you think they have 'forgotten')