If I was you I would try to get a settlement agreement
This is not good advice (sorry), not least of all due to the stress involved in the OP to have to convince their Employer that they have done something unlawful (when they haven't, based on information given).
Bullying is not 'against the law' in U.K., and there has been no harassment (the act of targeting a protected characteristic) that might give cause to @AdelaideD's employer to want to mitigate against a tribunal claim. This is often why Employers try to minimise bullying (even when they state in policy they take it seriously).
@AdalaideD - If your employer does have an anti-bullying/harassment Policy, it's worth pressing the point home that they are shirking their own stated responsibilities by not addressing the matter responsively - they sound utterly inept. It is typical of companies that try to kick it into the long grass rather than confronting the matter of bad behaviour by staff.
As an example, my organisation's message about anti B&H is prominent, and they do address it promptly if there is sufficient evidence. It makes a massive difference to how (the majority of) people treat each other when they know there could be repercussions on them if there are reports made of bad behaviour. A few nasties will just about 'stay this side of the line' (the excuse being "oh Person X doesn't suffer fools gladly". Um no they are a nasty unpleasant person!)
Defining what constitutes bullying is very tricky, and to some extent subjective, so it is worth highlighting "Person A did X and they made me feel Y" when describing the circumstances. Words such as humiliate, isolate, shame, threatened, frightened, can be powerful when trying to convey the daily challenges of someone targeting unwanted behaviours on you.