If this is a one-off for your friend, then it is extremely unlikely that the Inland Revenue will even find you in terms of tax liability for something as minor as this, so you can invoice your friend and not declare your earnings.
If however, this is going to be a regular thing, and the monies are beyond simple 'helping a friend' level, then you should think about self-employment status.
However to make the time end effort of self-employment and competing a tax return, you need to be earning significant amounts - and from your post it sounds like you are not at that level.
I'd invoice your friend, and bank the money and not worry about it.