This is currently an internal matter between you and the university. While there is a legal dimension to it and instructing a solicitor now may add some value to this matter for you, equally, involving a solicitor at cost may inflame matters further between you and the university.
For now, I suggest that you attend the investigation hearing (accompanied by a fellow-student to observe the hearing and take notes if the university investigation board allows) and present your defence to the charges of plagiarism as best you can and admit to any unintentional mistakes. You should review in detail the university’s internal policies regarding plagiarism, misconduct, investigation hearings and disciplinary matters against students and familiarise yourself with it prior to the hearing.
Do not get angry or upset with the university as they will only use it against you at a later date and you may say or do something that will prejudice your defence and damage your credibility in the eyes of the board.
If the university find against you and take disciplinary action against you in the form of ejecting you from your course and the university, you MAY have claims for Defamation and Breach of Contract for which you may pursue the university for damages in the form of a refund on your course fees paid and the damage to your reputation.
I recommend that you await the outcome of the hearing and then seek legal advice upon consideration of the result if it goes against you.
You may also lodge a formal complaint with the independent regulator and student advisory body for higher education providers. These are:
The Office for Standards in Education, Children’s Services and Skills (“OFSTED”): https://www.gov.uk/government/organisations/ofsted;
The Education and Skills Funding Agency: https://www.gov.uk/government/organisations/education-and-skills-funding-agency; and
The Office of the Independent Adjudicator: https://www.oiahe.org.uk/.