Having recently taken some professional advice on this, be mindful that HMRC are supposedly irritated by "service companies" who in reality only service one organisation. Their argument is that this makes you to all intents and purposes an employee and may try to argue this to the individual who "runs" the company. However, if you are supplying or servicing more than one customer this is conversley a strong postion to be in.
If your company was engaged by your customer via an agency such as Wynnwith / Brookfield etc. the risk to you of HMRC testing is diminished, as it is the agency who will cop the flak so to speak.
Maybe you have a case in that since you are already in PAYE employment by an employer, you have to be self employed for additional income? I don't know.
Not teaching you to suck eggs but is your existing employer happy for you to be working part time elsewhere? This usually has Contract of Employment and Flight Time Limitations issues.
If you are already paying tax in the higher bracket then having a company set up for additional income will mitigate your tax liability to (I think) 23%; act as a sole trader and you are liable for 40% of all your extra income (less your expenses).
I'm not professionally qualified in matters of taxation but what I've said above is what I remember from a professional meeting some weeks ago.
Good luck with your new venture though.