If you decide to cancel, you’ll definitely want to do so before the month is over otherwise you’ll get charged the regular rate.
Here’s the small print.
For a limited time only, new subscribers to the OnLive PlayPack Bundle (“PlayPack”) may purchase their first month of PlayPack for £2.00. After the one month promotional period, your PlayPack subscription will automatically renew at a rate of £6.99 per month. You may cancel your subscription at any time. Upon cancellation, your access to PlayPack ends immediately and no refund or credit is available for the remaining portion of the month. Offer and PlayPack subscription are not transferable. Offer only valid within the United Kingdom. Games are available for play only while connected to the OnLive Game Service. Games available in PlayPack are subject to change without notice. No retroactive discounts. Cannot be combined with other offers. No refunds. Offer has no cash value.
This would be a great way to try the cloud gaming service. Or you could use the 30 days to beat the games they have that you’re interested in.