Towards the start of this year, we had to fire someone I was managing due to (among other things) poor performance. We/I offered to supply a reference for ongoing employment. Their new company (I understand they've signed a contract already, for a somewhat related role) has now reached out for one, and addition to the usual questions about confirming employment dates they also have a bunch of questions about rating candidate performance, would I hire them again, etc. I gather from the form that these are standard questions for their process but I don’t think either me or the former employee were expecting it.

Constraints:

1: I don’t want to lie

2: I don't want to be mean to the former employee, while I don't think they were very good I have no animosity towards them

3: I don't think I can easily dodge this since last week they reached out to confirm that I was still OK to do this (obviously this was before I saw the form the new employer would send)

4: It's hard to write something generic because the employer's form is not free text but rather has specific questions as outlined above

5: I obviously don't want to do anything that could get my company (or me, I guess) into legal trouble - this is UK if that's relevant.

I have already asked my manager for advice and also written to our HR, plus asked friends/family who are not involved in the situation. I've yet to hear back from HR but a common suggestion has been to write to the former employee and confirm:

1: What they said about their departure from our company to the new employer, so as not to cause any problems.

2: Given the above constraints, whether they still want me to provide a reference (although if I don't I'm not sure what the new employer would make of it).

What would be the best course of action here?