NOW TV charges £9.99 for a one-day Sports Pass, £14.99 for a one-week one and £33.99 for a one-month pass:
https://www.nowtv.com/sports-purchase
That gives you unlimited access to all Sky Sports channels for the relevant period. (There's no option just to take the F1 channel).
As Tuvok indicates, you can watch motor sport (including F1) for free on Germany's RTL channel online if you use a VPN. (PureVPN has a good reputation and actually promotes its services to access F1 on RTL:
https://www.purevpn.com/ ). If you're German isn't up to it, you can mute the sound and listen to the commentary on BBC Radio 5 Live.