Yes I can understand. That is why I advise for now, to give them the TA standards (cross off any that are not relevant to their role as LSA) and have them self-assess against them.
Ideally, they would provide physical evidence to prove the grades they give themselves. How long do you have for this process?
If you have the next 4 weeks say, put it into a table and present it to them, accepting them to grade themselves [not met/met/above expectations] with at very least, bullet points of how they feel they have done so.
Then they bring this to you in a 1:1 appraisal meeting and discuss them. If you agree with their presentation of each standard, then confirm the gradings they have given themselves. If not, you must be clear as to why [which could be because it isn’t evidenced] and inform them of the grading you will give instead.
You then type up the meeting with a comment at the end, stating their overall performance based on this.
Send it them, so that they have 24 hours to read your statement and make a responding comment also [they do not have to agree with what you have said. As their line manager, you have a wider perspective as of course, what one person thinks is exceeding, may be another person’s just met].
Then you would both sign the copies, having 1 for them, 1 for you, 1 for your boss, and 1 for HR.
Does this help?