For tax purposes you need to earn over your personal allowance but if you already have a full time job and are earning more than £6470 per year in that you're already over your personal allowance and therefore need to declare it.
As for pricing, it's not nice but phone other photographers and ask what they charge, easiest way to find out, like I said on a moral footing it isn't good though
Or, forget that and just work out how much you need to sort their book of images out plus some money for petrol and whatever then on top of that price your next purchase, lens, tripod, photoshop ? and there is your figure.
No one can tell you how much or how little you should charge, it's your time therefore you have to decide. Always difficult if it's for a friend though.