I suggest that, while it's fresh in your mind, you get a statement of the story so far written down in as much detail as possible. You can then hand over a copy of this statement at a disciplinary hearing.
Include as much as you can of the following:
When and how you first became aware that the job applicant didn't have a work visa.
The date that the person started working for your employer. Include their name, their job and any other relevant details.
You say that, "I told my line manager, as soon as I realised what had happened." So, I assume that this was a conversation and not an email. So, write down everything you remember about this conversation; date, time, place, what you said, what your line manager said etc. ( FYI, this was the 1st protected disclosure. )
Write down anything relevant to this matter that happened between that conversation and the date that you reported it to the Home Office/Border Agency.
The details of your report of illegal working to the Home Office/ Border Agency, ( The 2nd protected disclosure ) as far as you can remember them.
The details of the text message that you sent to your manager telling her what you'd done. ( The 3rd protected disclosure. )
Describe what your employer did next. The suspension, the threat/pursuit of discipinary action etc. Include dates and references to any documents they sent/gave to you.
Document in detail the effect of their actions on your health and feelings. (This will be used to determine the £amount of an 'injury to feelings' award or settlement.)
You should contact your employer requesting access to your work emails (and other documents?). Tell them that you need this to prepare your defence to their disciplinary charges.
They shouldn't refuse you this access, although they may insist that it's supervised. However, if they do refuse, or if you discover that the emails have been deleted it could well be because they have been using your absence on suspension to remove evidence of their wrongdoing.
Have you been given a date for the disciplinary hearing?
If not, there's no telling how long your employer will choose to leave you in this state of limbo.
If you want to hurry things along you could submit a formal grievance, using a copy of the statement above, and stating that you consider that your employer has subjected you to victimisation discrimination by way of 2 detriments, i.e. 1. suspension and 2. the threat/pursuit of disciplinary action, on the grounds that you made protected disclosures.