I need a UK lawyer/solicitor who specialises in defamation cases

In 2022 paperwork was submitted without my knowledge to HMRC which allowed a company to receive tax rebates on my behalf. After requesting copies of the paperwork it was a fraudulent signature. I’ve never enquired or asked any third party to act on my behalf with HMRC. After doing research I found hundreds of others affected and news articles about this company. After joining a private Facebook group set up for victims of this company I told other members to complain to HMRC and leave a negative review on TrustPilot about this same company. Yesterday I received paperwork from a solicitor saying they’re going to sue me for defamation unless I remove all comments and withdraw my complaint to HMRC and sign paperwork saying I actually authorised this company to act on my behalf (false)