Expenses claimed in the UK must be “wholly and exclusively for the purposes of the business” and must also be incurred ‘necessarily’ in the performance of the duties of the employment.
Your contract work has employment status in the UK so expenses should not be confused with those allowable for self-employment persons
Expenses are claimed on a monthly arising basis and cannot be rolled over across tax years. Any historic expenses unclaimed can be filed through your self-assessment (UK tax return).
Most expenses are claimed as actuals with a few exceptions:
Out of base, temporary and training related travel and subsistence (HMRC scale rates may apply)
An overview of expenses allowable in the UK is attached