The tax free “Christmas Party Allowance” is £150 per head per annum.
It can be spread over as many annual events as you like, but each one must be annual and an event, arranged by and billed to the company. The total party cost to the company, including taxis, etc. is included in the calculation, apportioned equally between partygoers. There is no part paying/refunding allowed. If this limit is exceeded then one event becomes taxable on staff members as a benefit in kind, and if they were accompanied by a personal guest then the cost of entertaining that guest also becomes taxable on that member of staff.