Hi
@TC seeker,
That's a great question.
Generally, you are not allowed to work full-time during term time at your university. Based on what you have described, it appears to me that the time when you are writing your dissertation in Term 3 will still fall under term time under your visa restrictions.
Therefore, if possible, I would try and obtain a vacation/ work permission from your university for the vacation scheme dates. When I was completing my first Master's and applying to vacation schemes/ TCs at the same time, I had the option of requesting a 'vacation leave' in order to complete a work placement, such as a vacation scheme.
I hope this helps!