Hi
I am not an accountant but have been through this in the last 12 months. I took professional advice and I'd recommend you do the same, but my understanding is that as you are a pilot (from your profile), as opposed to say a brain surgeon with a pplh, you can justifiably argue that the cost of the extra ratings/training is being incurred to enable you to increase your earnings going forward and are therefore a valid business cost.
An LLP (limited liability partnership) is more flexible than an Ltd - you get the limited liability of an Ltd but can offset any losses against other income, including historic income whilst you can only offset losses in an Ltd against profits in the Ltd. But you need at least one partner you can trust in the LLP as you can't form one alone. Apart from that you start paying a self employed stamp (£10/mth) and you'll have to file partnership accounts, file annually at company house, file quarterly VAT returns if you decide/are eligible to file for VAT registration which all costs, so you need to be sure that you'll save more tax. With helicopter training there shouldn't be much problem there!
But its worth talking to an accountant, dependant on your circumstances you may be able to simply claim the costs against your current earnings without setting up a company, that solution just made sense for me.
W