Events held for clients that are free of charge are treated as business entertainment thus disallowed for tax purposes. If the company is holding and an event but is charging the attenders for their attendance, it could be tax deductible.

For further info see HMRC manuals:

https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim45012

https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim45013

https://www.gov.uk/hmrc-internal-manuals/business-income-manual/bim45014