The only way to get ITV Encore without a Sky subscription is to buy a Now TV box. Argos will sell you one for £14.99, which includes a 3-month Entertainment Pass:
http://www.argos.co.uk/product/4146678
As well as ITV Encore, your Pass will get you Sky 1, Sky Arts, Sky Living, Sky Atlantic, Gold, Comedy Central, Fox, MTV, Discovery Channel and Nat Geo Wild. There are also box sets available to watch. If you want to keep receiving those services after the three months are up you'll pay £6.99 per month. (There's no contract. You can opt out or in at any time).
There are also separate passes available for access to Sky Sports, Sky Cinema or kids' channels.
Even without a valid pass a Sky box makes a 'non-smart' TV into a 'smart' one by providing access to catchup services and things like Youtube on your telly.