All I have done is talk about age regression to colleagues that ask and i have shown them images of Little at play.
The problem with this is that it may have made one of your colleagues feel uncomfortable, which may have caused them to mention it to the new manager, or maybe a busy body heard about it or overheard your colleagues talking about it which caused them to mention it to your manager.
Non of the images are pornographic or show nudity.
They may not be but since littles are linked to BDSM by many vanillas they still class it as kink and fetish and therefore inappropriate.
However,
You do have a right to a private life, so if they do discriminate against you purely on the basis of your private social media, providing the companies name has not been mentioned anywhere that could be linked to ABDL or your tumblr profile, then they may be breaching your right to a private life which is a breach of both the human rights act and UK law.
Employers do have a right to monitor communications in the workplace, however, they must make you aware that you are being monitored beforehand.
You have the right to see any information held about you, like emails or CCTV footage.
However, again your right to a private life means you do have the right to some privacy in the workplace, if your employer fails to acknowledge this then again, they could be in breach of both the human rights act and UK law.
Also, please check the disciplinary procedure outlined in your contract, an employer MUST follow this procedure before they can dismiss you, failure in following this procedure would allow you to pursue them for unfair dismissal providing you have been employed for 2 years.
Also, I would check your companies social media and internet policies.
I am a UK law student however I would also consult a solicitor depending on how the meeting goes on Monday, good luck!