If you pay for an aircraft rating before you join a company, forget any chance of tax relief.
Not strictly true, I used ot be self employed and claimed tax relief on my type ratings, and the upgrade exams from CPL to ATPL.
The tax office I called also confirmed that a self employed pilot can claim the tax back on a rating.
So if you are thinking of doing a rating out of your own pocket, do it as a self employed person.