If you simply let your Sky subscription lapse (but retain the box) you'll still be able to receive all of these free channels via your satellite dish:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
That list is sufficient for many people's needs but it doesn't have certain channels which are only available free of charge via (terrestrial) Freeview. In particular, you would no longer be able to receive the free UKTV channels (such as Dave, Yesterday and Drama) via your satellite dish. You'd also need to have a terrestrial aerial. Then (assuming that your TV set has built-in Freeview, which almost every set sold does anyway) you'll be able to receive these channels (which include the free UKTV ones) via your conventional aerial:
http://www.freeview.co.uk/whats-on/channels
Putting those two lists together shows you all of the channels that are available free of charge without your Sky subscription. However you can get back many of the channels that a basic Sky subscription provides (such as Sky 1, Sky Atlantic, Sky Living, Gold, Comedy Central, ITV Encore, Fox, MTV, Sky Arts 1 and Discovery, plus some kids channels) for £6.99 per month by using a NOW TV box to receive them via the internet:
http://www.nowtv.com/box
The box costs you £19.99 online but you get 3 months access to the Entertainment package included, so it's effectively free. (Some supermarkets sometimes offer them even cheaper). The box also turns any TV into a 'smart' one, so that you can watch catch-up services (or Youtube videos, etc) on your telly. There's no subscription commitment. (The 'smart' capability will still work even if you stop paying).
NOW TV boxes are actually provided by Sky and offer the cheapest way to access some of their most popular paid-for services.