First letting - legal costs: The accepted view is that legal costs for drawing up a lease and other expenses, e.g. surveyor fees, associated with the first time you let a property count as capital expenditure and so are not tax deductible. This is because the lease only puts you in a position to start your rental business rather than run it. However, HMRC's view is that where the first let is for a period of no more than one year, the legal etc. expenses can be deducted for tax purposes from rental income. Even where the expenses aren't deductible for a first let because it's for longer than a year, they will be for the lease renewal if that's for less than 50 years and doesn't include a lump sum payment (usually referred to as a premium).
Deductible travel: If you make a journey that is wholly and exclusively for the purpose of the rental business, for example, to make a repair to a property or to check its condition, you can claim the cost of travel from the place you manage the letting (often that will be your home) to the rental property and back.
If you use your car you can claim either a proportion of your motor expenses or HMRCs' mileage allowance (the latter is simpler to work out and often more generous).
Capital allowances: Whilst you can't claim Capital Allowances for equipment for use in the property by tenants you can claim Capital Allowances for equipment you buy to run your rental business. For example, tools with an expected life of more than two years, which you use to maintain your let properties or a computer if that's what you use to administer and manage your rental business. However, you must limit your claim proportionately to account for any non-business use.