You will need to include the foreign pages (SA106) which will allow you to declare the Japanese income and tax paid.
You would also need to complete SA102 (employment) or SA103 (self employment) or their online equivalents.
Self Assessment tax return forms
It is not clear whether you are employed by the university to give the lectures or self employed. Guidance on expenses if employed can be found here:
EIM34003 - Travelling expenses: employees working abroad: expenses that may be deducted: example
and if self employed, here:
BIM47712 - Specific deductions: travel and subsistence: living expenses abroad — foreign trades