Travel expenses: general: overseas conferences, seminars and study tours: apportionment
Sections 337 to 339 ITEPA 2003If you accept that a particular overseas trip has been made necessarily to carry out the duties of the employment you may still find that some expenses are not deductible. For example, a short break may be taken during an overseas conference.
You should not accept an apportionment of the total cost of the trip on a percentage basis, calculated by looking at the relative time spent on business and non-business activities. There is no basis in law for such an apportionment, see EIM31660. Instead you should examine the expenses individually and permit a deduction only for those that are necessarily incurred.
The correct approach is illustrated by example EIM31811.