You can also consider VAT registration if you have not already done so and reclaim the VAT on your training. You will have to charge VAT on your invoice as self-employed instructor (whether as sole trader or ltd company).
www.hmce.gov.uk - and download the forms for HM Customs & Excise.
There is nothing particularly complex in VAT or taxation (or Company Law for that matter) - if you can pass your ATPL exams, you can understand the concepts behind tax! There are rules and both the Inland Revenue and Customs & Excise are helpful when people are starting out.
Cheers
Whirlygig
PS - you don't NEED a separate bank account to be self-employed, although you should for a limited company but, if you don't have a separate account, you have to be meticulous in keeping your records.