Any standard deductions allowed for administration expenses?

You don’t indicate if your operating as a company or an individual? Nowadays I’m incorporated and use an accountant as it’s getting way too complex for me to be bothered with, (old age creeps on).

However, from memory, all of those you list above are claimable, but more as individual items than under a coverall umbrella. I’m afraid you will need to keep a spreadsheet and receipts incase you get chosen for an inspection.

You may benefit from pulling all the information together and then accessing an accountant for a tutorial as to how to enter it all appropriately. That said, I always found the tax office was very helpful on the phone.

If you work from home just one day a year you can claim a ‘standard’ £6/week towards heating etc.

For mileage there’s a standard pence per mile rate for fuel & vehicle costs but you must keep a diary record of journeys and distance travelled. https://www.gov.uk/government/publications/rates-and-allowances-travel-mileage-and-fuel-allowances

As far as I know everything else has to be for business use only (and retain receipts). For example, if you use the same mobile for personal & letting, you can only claim the cost of the calls not covered by your monthly allowance. If you have 2 mobiles and one is dedicated for letting you can deduct it’s cost – but must not then use it for a personal call.

I just claim £26 per annum for each property I let. Works out at only 50p per week to cover phone calls and postage. IMO unlikely to be challenged by HMRC

Any expense you claim should be ‘wholly and exclusively’ for the business. So if you claim mileage at 45p a mile going to your property, maybe via the diy store, that is claimable but it you drop into your property on the way to visit your Mum, it is not!