Prior to flight training I was self-employed and VAT registered.
Now, if I needed to buy any tools or when I purchased my van, I was able to claim the VAT back and offset the cost against my tax. I don't see how buying your FI rating is any different. You are purchasing something that will enable you to do your job.
Other expenses you can put through the tax are clothes - uniform, petrol to get to and from work, charts and other equipment you need to do your job.
I know others have advised not to get an accountant invloved, but a shrewd accountant will enable you to pay as little tax as possible and will know any loop holes you can exploit to get some of that hard earned cash back you have spent during training. My accountant used to charge me £350 at the end of each tax year, not a great deal on the grand scheme of things.