Expenses incurred while on assignment should be claimed back via your company, rather than your client.
Because you are now an employee of your own company, you should claim all allowable expenses from your own company, just as you did in your former employment. To use HMRC terminology, these expenses must have been incurred “wholly, exclusively and necessarily” for the purpose of the business, and we will be able to guide you as to what expenses are allowable.
It is acceptable for your client to pay certain allowable expenses on your behalf, such as air fares and hotel accommodation, directly to suppliers but your client should not reimburse you personally for any expenses that you have incurred.
The amount you claim from your own company will often not be the same as the amount invoiced to your client, since for example there may be additional marketing or administration expenses, differences in mileage rates (you may be claiming more, or less, than the allowable HMRC rate from your client), or some of the expenses you claim from your company may be included in the daily rate that you charge your client.
If working under IR35, you cannot claim for travel and subsistence relating to journeys to and from work. You may only claim for travel relating to one off occasional journeys such as a business meeting at another office that you do not usually visit. If you are working exclusively under IR35, your ability to pay any other allowable expenses to yourself in full may be occasionally be restricted, and so at this stage you may need to loan funds to the company so that the company can pay you. Now that you are working, it is better to loan personal money to the company so that you can pay yourself, and thus crystallise these expenses as company expenses. If you later receive some nonIR35 income you will be able to repay your loans.