Is this something that would be valued in Poland?
If it is enough practical knowledge, skills included, for example first steps for searching a job in UK, starting new business, ability to fulfill official letters, papers, etc - the answer is yes.
Is there are market for this?
Since the UK borders are still open, and new employers are yet welcomed there - yes. Lot of young people see their future abroad.
Would it be beneficial to provide a more intensive full time course over 10-14 weeks or a part time course over 20-30 weeks?
Full time course with guaranteed job / internship would be best solution.
Would Polish schools welcome the opportunity to assist in the training through the offer of school placement for students on this proposed course?
It depends how many students would volunteer, and how the school could benefit. Good business plan required.