Is a company's financial year and accounting period the same thing?
In short no, the two things are different.
The financial year of company is determined initially by the date your company is incorporated with Companies House. It's accounting period for corporation tax is instead decided by HMRC for the preparation of the company tax return and paying corporation tax.
The accounting period doesn't start until your company starts trading - i.e. it becomes 'active'.
Your company may start trading the day it is incorporated and as a result your company's financial year and accounting period will start on the same day and so be the same. However, if your business doesn't become 'active' straight away, your accounting period for corporation tax will probably start at a later date.
At the point when your company becomes active, you must inform HMRC within three months. HMRC will then approve the start date of your corporation tax accounting period. It will finish on your accounting reference date, exactly like the annual accounts.